feat(transfer): expose GUI download segments knob

GTC-5 wires the user-facing intra-file range parallelism preset
into the GUI download path. Settings > Transfers gains a new
"Download Segments per File" select (Auto / 2 / 4 / 8 / 16) right
after Concurrent Transfers, persisted in the app_settings vault
entry under `downloadSegments` (default 0 = Auto).

The resolved value is forwarded to `provider_download_file` and
`provider_download_folder` via the existing `download_segments:
Option<u32>` parameter that the backend already accepts (see
provider_commands.rs:911 / :1281 / :1112 eligibility gate). Auto
maps to `undefined` on the frontend, which the Rust handler
treats as the legacy single-stream behaviour.

Scope:
- IN: provider_download_file, provider_download_folder
  (provider transport path: SFTP, S3, HTTP-based cloud).
- OUT: rclone_crypt_provider_download_file / _folder. The
  crypt overlay reads the full ciphertext via download_to_bytes
  before decrypting in memory; it does not use provider.download()
  streaming, so the segmented helper would require a separate
  refactor (segmented download to .aerotmp, then streaming
  decrypt). Intentionally deferred for a later slice.
- OUT: download_files_batch (FTP GUI batch on FtpDownloadExecutor)
  stays `download_segments: None` per the no-double-pool invariant
  already documented in lib.rs:4003 and the three sibling OAuth
  fallback sites.

Translations: 4 new keys (settings.downloadSegments,
downloadSegmentsAuto, downloadSegmentsN, downloadSegmentsDesc) in
all 47 locales, npm run i18n:validate clean (0 warnings, 0
[NEEDS TRANSLATION]). `{n}` placeholder preserved literally;
Armenian uses native Mesropian script.

Validation:
- npm run typecheck: green.
- cargo check: green (no Rust changes).
- Live SFTP @ axpbuntu, 64 MiB:
  seg=1 (Auto) 34.05s vs seg=4 11.41s, speedup 2.98x,
  byte-identical YES
  (cargo test --test integration_gtc_wan_segmented
   gtc_gui_sftp_segmented_byte_identical_vs_single_stream
   -- --ignored --nocapture --test-threads=1).

Co-Authored-By: aeroftp[bot] <aeroftp[bot]@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: 3 people

src/App.tsx

@@ -371,6 +371,7 @@ const App: React.FC = () => {
     compactMode, showHiddenFiles, showToastNotifications, confirmBeforeDelete,
     showStatusBar, defaultLocalPath, fontSize, fontFamily, doubleClickAction, rememberLastFolder, visibleColumns,
     sortFoldersFirst, showFileExtensions, timeoutSeconds, maxConcurrentTransfers, retryCount,
+    downloadSegments,
     fileExistsAction, swapPanels,
     showActivityLog, showConnectionScreen,
     showSettingsPanel, setShowSettingsPanel, setShowConnectionScreen,
@@ -5811,6 +5812,9 @@ interface UpdateVerificationInfo {
               maxConcurrent: effectiveMaxConcurrentTransfers,
               retryCount,
               timeoutSeconds,
+              // GTC-5: intra-file range parallelism per download.
+              // 0 = Auto (backend = legacy single-stream).
+              downloadSegments: downloadSegments > 0 ? downloadSegments : undefined,
             });
             // pwd is restored by the backend (provider_download_folder saves/restores pwd)
           } else {
@@ -5875,7 +5879,14 @@ interface UpdateVerificationInfo {
             });
           } else if (isProvider) {
             // Use provider command for file download
-            await invoke('provider_download_file', { remotePath: remoteFilePath, localPath: localFilePath, modified: remoteModified });
+            await invoke('provider_download_file', {
+              remotePath: remoteFilePath,
+              localPath: localFilePath,
+              modified: remoteModified,
+              // GTC-5: intra-file range parallelism per download.
+              // 0 = Auto (backend = legacy single-stream).
+              downloadSegments: downloadSegments > 0 ? downloadSegments : undefined,
+            });
           } else {
             const params: DownloadParams = { remote_path: remoteFilePath, local_path: localFilePath, modified: remoteModified };
             await invoke('download_file', { params });

src/components/SettingsPanel.tsx

@@ -213,6 +213,8 @@ interface AppSettings {
     // Transfers
     maxConcurrentTransfers: number;
     retryCount: number;
+    /** Intra-file range parallelism per download. 0 = Auto. */
+    downloadSegments: number;
     // File Handling
     fileExistsAction: 'ask' | 'overwrite' | 'skip' | 'rename' | 'resume' | 'overwrite_if_newer' | 'overwrite_if_different' | 'skip_if_identical';
     preserveTimestamps: boolean;
@@ -256,6 +258,7 @@ const defaultSettings: AppSettings = {
     ftpMode: 'passive',
     maxConcurrentTransfers: 5,
     retryCount: 3,
+    downloadSegments: 0,
     fileExistsAction: 'ask',
     preserveTimestamps: true,
     transferMode: 'auto',
@@ -2831,6 +2834,18 @@ export const SettingsPanel: React.FC<SettingsPanelProps> = ({ isOpen, onClose, o
                                             </select>
                                         </div>
 
+                                        <div>
+                                            <label className="block text-sm font-medium mb-1">{t('settings.downloadSegments')}</label>
+                                            <select value={settings.downloadSegments} onChange={(e) => updateSetting('downloadSegments', parseInt(e.target.value))} className="w-full px-3 py-2 border border-gray-300 dark:border-gray-600 rounded-lg bg-white dark:bg-gray-700 text-gray-900 dark:text-gray-100 text-sm">
+                                                {[0, 2, 4, 8, 16].map((n) => (
+                                                    <option key={n} value={n}>
+                                                        {n === 0 ? t('settings.downloadSegmentsAuto') : t('settings.downloadSegmentsN', { n })}
+                                                    </option>
+                                                ))}
+                                            </select>
+                                            <p className="text-xs text-gray-500 mt-1">{t('settings.downloadSegmentsDesc')}</p>
+                                        </div>
+
                                         <div>
                                             <label className="block text-sm font-medium mb-1">{t('settings.retryCountOnError')}</label>
                                             <select value={settings.retryCount} onChange={(e) => updateSetting('retryCount', parseInt(e.target.value))} className="w-full px-3 py-2 border border-gray-300 dark:border-gray-600 rounded-lg bg-white dark:bg-gray-700 text-gray-900 dark:text-gray-100 text-sm">

src/hooks/useSettings.ts

@@ -80,6 +80,9 @@ export interface AppSettings {
   timeoutSeconds: number;
   maxConcurrentTransfers: number;
   retryCount: number;
+  /** Intra-file range parallelism per download. 0 = Auto (backend decides,
+   *  today: single-stream). Supported values: 0, 2, 4, 8, 16. */
+  downloadSegments: number;
   fileExistsAction: 'ask' | 'overwrite' | 'skip' | 'rename' | 'resume' | 'overwrite_if_newer' | 'overwrite_if_different' | 'skip_if_identical';
   swapPanels: boolean;
   lastLocalPath?: string;
@@ -116,6 +119,7 @@ const DEFAULTS: AppSettings = {
   timeoutSeconds: 30,
   maxConcurrentTransfers: 5,
   retryCount: 3,
+  downloadSegments: 0,
   fileExistsAction: 'ask',
   swapPanels: false,
   disableUpdateChecks: false,
@@ -145,6 +149,7 @@ export const useSettings = () => {
   const [timeoutSeconds, setTimeoutSeconds] = useState(DEFAULTS.timeoutSeconds);
   const [maxConcurrentTransfers, setMaxConcurrentTransfers] = useState(DEFAULTS.maxConcurrentTransfers);
   const [retryCount, setRetryCount] = useState(DEFAULTS.retryCount);
+  const [downloadSegments, setDownloadSegments] = useState(DEFAULTS.downloadSegments);
   const [fileExistsAction, setFileExistsAction] = useState<AppSettings['fileExistsAction']>(DEFAULTS.fileExistsAction);
   const [swapPanels, setSwapPanels] = useState(DEFAULTS.swapPanels);
   const [disableUpdateChecks, setDisableUpdateChecks] = useState(DEFAULTS.disableUpdateChecks);
@@ -174,6 +179,9 @@ export const useSettings = () => {
     if (typeof parsed.timeoutSeconds === 'number') setTimeoutSeconds(parsed.timeoutSeconds);
     if (typeof parsed.maxConcurrentTransfers === 'number') setMaxConcurrentTransfers(parsed.maxConcurrentTransfers);
     if (typeof parsed.retryCount === 'number') setRetryCount(parsed.retryCount);
+    if (typeof parsed.downloadSegments === 'number' && [0, 2, 4, 8, 16].includes(parsed.downloadSegments)) {
+      setDownloadSegments(parsed.downloadSegments);
+    }
     if (
       typeof parsed.fileExistsAction === 'string' &&
       ['ask', 'overwrite', 'skip', 'rename', 'resume', 'overwrite_if_newer', 'overwrite_if_different', 'skip_if_identical'].includes(parsed.fileExistsAction)
@@ -269,6 +277,7 @@ export const useSettings = () => {
     timeoutSeconds,
     maxConcurrentTransfers,
     retryCount,
+    downloadSegments,
     fileExistsAction,
     swapPanels,
     disableUpdateChecks,
@@ -298,6 +307,7 @@ export const useSettings = () => {
     setTimeoutSeconds,
     setMaxConcurrentTransfers,
     setRetryCount,
+    setDownloadSegments,
     setFileExistsAction,
     setSwapPanels,
     setDisableUpdateChecks,

src/i18n/locales/bg.json

@@ -998,7 +998,11 @@
             "bridgeUnsupportedProto": "{protocol} не се поддържа от {app}",
             "bridgeSecretFull": "Идентификационните данни се възстановяват и повторно се шифроват в хранилището на AeroFTP.",
             "bridgeSecretLimited": "Само част от идентификационните данни може да бъде възстановена; може да се наложи да въведете отново някои тайни.",
-            "bridgeSecretMetadata": "Само метаданни за връзката: тайните остават в ключодържателя/агента на {app} и трябва да бъдат въведени отново."
+            "bridgeSecretMetadata": "Само метаданни за връзката: тайните остават в ключодържателя/агента на {app} и трябва да бъдат въведени отново.",
+            "downloadSegments": "Сегменти за изтегляне на файл",
+            "downloadSegmentsAuto": "Auto (единичен поток)",
+            "downloadSegmentsN": "{n} сегмента на файл",
+            "downloadSegmentsDesc": "Разделя всяко изтегляне на паралелни байтови диапазони. Auto запазва текущото поведение с единичен поток. Действа при доставчици, които поддържат range заявки (SFTP, S3, HTTP) и при файлове, достатъчно големи, за да има полза."
         },
         "devtools": {
             "title": "AeroTools",

src/i18n/locales/bn.json

@@ -998,7 +998,11 @@
             "bridgeUnsupportedProto": "{protocol} {app} দ্বারা সমর্থিত নয়",
             "bridgeSecretFull": "শংসাপত্রগুলো পুনরুদ্ধার করে AeroFTP ভল্টে পুনরায় এনক্রিপ্ট করা হয়।",
             "bridgeSecretLimited": "শংসাপত্রের শুধুমাত্র একটি অংশ পুনরুদ্ধার করা যাবে; আপনাকে কিছু গোপন তথ্য পুনরায় লিখতে হতে পারে।",
-            "bridgeSecretMetadata": "শুধুমাত্র সংযোগ মেটাডেটা: গোপন তথ্য {app}-এর কীচেইন/এজেন্টে থেকে যায় এবং পুনরায় লিখতে হবে।"
+            "bridgeSecretMetadata": "শুধুমাত্র সংযোগ মেটাডেটা: গোপন তথ্য {app}-এর কীচেইন/এজেন্টে থেকে যায় এবং পুনরায় লিখতে হবে।",
+            "downloadSegments": "প্রতি ফাইলে ডাউনলোড সেগমেন্ট",
+            "downloadSegmentsAuto": "Auto (একক স্ট্রিম)",
+            "downloadSegmentsN": "প্রতি ফাইলে {n} সেগমেন্ট",
+            "downloadSegmentsDesc": "প্রতিটি ডাউনলোডকে সমান্তরাল বাইট রেঞ্জে ভাগ করুন। Auto বর্তমান একক-স্ট্রিম আচরণ বজায় রাখে। যেসব প্রোভাইডার রেঞ্জ রিকোয়েস্ট সমর্থন করে (SFTP, S3, HTTP) এবং ফাইল যথেষ্ট বড় হলে কার্যকর।"
         },
         "devtools": {
             "title": "AeroTools",

src/i18n/locales/ca.json

@@ -998,7 +998,11 @@
             "bridgeUnsupportedProto": "{protocol} no és compatible amb {app}",
             "bridgeSecretFull": "Les credencials es recuperen i es tornen a xifrar al vault d'AeroFTP.",
             "bridgeSecretLimited": "Només es pot recuperar una part de les credencials; pot ser que hagis de tornar a introduir alguns secrets.",
-            "bridgeSecretMetadata": "Només metadades de connexió: els secrets es queden al clauer/agent de {app} i s'han de tornar a introduir."
+            "bridgeSecretMetadata": "Només metadades de connexió: els secrets es queden al clauer/agent de {app} i s'han de tornar a introduir.",
+            "downloadSegments": "Segments de descàrrega per fitxer",
+            "downloadSegmentsAuto": "Auto (flux únic)",
+            "downloadSegmentsN": "{n} segments per fitxer",
+            "downloadSegmentsDesc": "Divideix cada descàrrega en intervals de bytes paral·lels. Auto manté el comportament actual de flux únic. Efectiu en proveïdors compatibles amb sol·licituds de rang (SFTP, S3, HTTP) per a fitxers prou grans com per beneficiar-se'n."
         },
         "devtools": {
             "title": "AeroTools",

src/i18n/locales/cs.json

@@ -998,7 +998,11 @@
             "bridgeUnsupportedProto": "{protocol} není podporován v {app}",
             "bridgeSecretFull": "Přihlašovací údaje se obnoví a znovu zašifrují do trezoru AeroFTP.",
             "bridgeSecretLimited": "Lze obnovit jen část přihlašovacích údajů; některá tajemství bude možná nutné zadat znovu.",
-            "bridgeSecretMetadata": "Pouze metadata připojení: tajemství zůstávají v klíčence/agentovi {app} a musí být zadána znovu."
+            "bridgeSecretMetadata": "Pouze metadata připojení: tajemství zůstávají v klíčence/agentovi {app} a musí být zadána znovu.",
+            "downloadSegments": "Segmenty stahování na soubor",
+            "downloadSegmentsAuto": "Auto (jeden proud)",
+            "downloadSegmentsN": "{n} segmentů na soubor",
+            "downloadSegmentsDesc": "Rozdělí každé stahování na paralelní bajtové rozsahy. Auto zachovává současné chování s jedním proudem. Funguje u poskytovatelů, kteří podporují požadavky na rozsah (SFTP, S3, HTTP), a u souborů dostatečně velkých, aby z toho měly užitek."
         },
         "devtools": {
             "title": "AeroTools",

src/i18n/locales/cy.json

@@ -998,7 +998,11 @@
             "bridgeUnsupportedProto": "Nid yw {app} yn cefnogi {protocol}",
             "bridgeSecretFull": "Caiff y manylion adnabod eu hadfer a'u hailamgryptio i mewn i gladd AeroFTP.",
             "bridgeSecretLimited": "Dim ond rhan o'r manylion adnabod y gellir ei hadfer; efallai y bydd angen i chi ailosod rhai cyfrinachau.",
-            "bridgeSecretMetadata": "Metadata cysylltiad yn unig: mae'r cyfrinachau'n aros yng nghadwyn allweddi/asiant {app} a rhaid eu hailosod."
+            "bridgeSecretMetadata": "Metadata cysylltiad yn unig: mae'r cyfrinachau'n aros yng nghadwyn allweddi/asiant {app} a rhaid eu hailosod.",
+            "downloadSegments": "Segmentau lawrlwytho fesul ffeil",
+            "downloadSegmentsAuto": "Auto (un ffrwd)",
+            "downloadSegmentsN": "{n} segment fesul ffeil",
+            "downloadSegmentsDesc": "Rhannu pob lawrlwythiad yn ystodau beit cyfochrog. Mae Auto yn cadw'r ymddygiad un ffrwd presennol. Effeithiol gyda darparwyr sy'n cefnogi ceisiadau ystod (SFTP, S3, HTTP) ar gyfer ffeiliau digon mawr i elwa."
         },
         "devtools": {
             "title": "AeroTools",

src/i18n/locales/da.json

@@ -998,7 +998,11 @@
             "bridgeUnsupportedProto": "{protocol} understøttes ikke af {app}",
             "bridgeSecretFull": "Loginoplysningerne gendannes og krypteres på ny i AeroFTP-vaulten.",
             "bridgeSecretLimited": "Kun en del af loginoplysningerne kan gendannes; du skal muligvis indtaste nogle hemmeligheder igen.",
-            "bridgeSecretMetadata": "Kun forbindelsesmetadata: hemmelighederne forbliver i {app}'s nøglering/agent og skal indtastes igen."
+            "bridgeSecretMetadata": "Kun forbindelsesmetadata: hemmelighederne forbliver i {app}'s nøglering/agent og skal indtastes igen.",
+            "downloadSegments": "Download-segmenter pr. fil",
+            "downloadSegmentsAuto": "Auto (enkelt stream)",
+            "downloadSegmentsN": "{n} segmenter pr. fil",
+            "downloadSegmentsDesc": "Opdel hver download i parallelle byte-intervaller. Auto bevarer den nuværende adfærd med enkelt stream. Virker hos udbydere, der understøtter range-anmodninger (SFTP, S3, HTTP), for filer der er store nok til at have gavn af det."
         },
         "devtools": {
             "title": "AeroTools",

src/i18n/locales/de.json

@@ -998,7 +998,11 @@
             "bridgeUnsupportedProto": "{protocol} wird von {app} nicht unterstützt",
             "bridgeSecretFull": "Die Zugangsdaten werden wiederhergestellt und erneut in den AeroFTP-Vault verschlüsselt.",
             "bridgeSecretLimited": "Nur ein Teil der Zugangsdaten kann wiederhergestellt werden; einige Geheimnisse müssen Sie möglicherweise erneut eingeben.",
-            "bridgeSecretMetadata": "Nur Verbindungsmetadaten: Die Geheimnisse bleiben im Schlüsselbund/Agenten von {app} und müssen erneut eingegeben werden."
+            "bridgeSecretMetadata": "Nur Verbindungsmetadaten: Die Geheimnisse bleiben im Schlüsselbund/Agenten von {app} und müssen erneut eingegeben werden.",
+            "downloadSegments": "Download-Segmente pro Datei",
+            "downloadSegmentsAuto": "Auto (einzelner Stream)",
+            "downloadSegmentsN": "{n} Segmente pro Datei",
+            "downloadSegmentsDesc": "Teilt jeden Download in parallele Byte-Bereiche auf. Auto behält das bisherige Verhalten mit einem einzelnen Stream bei. Wirksam bei Anbietern, die Range-Anfragen unterstützen (SFTP, S3, HTTP), und bei Dateien, die groß genug sind, um davon zu profitieren."
         },
         "devtools": {
             "title": "AeroTools",