feat(translator): add configurable DeepLX-like backend

[RanderDouble]

Summary

This PR adds a minimal DeepLX-like API backend option to the Translator plugin.

Changes

• add a new DeepLX-like API backend in plugin settings
• add configurable API URL and optional API Key fields
• send custom backend translation requests as POST JSON
• attach Authorization: Bearer <token> when an API key is provided
• support parsing either data or translations[0].text from the response
• include backend and API URL in the cache key to avoid stale cached results
  after switching backends
• document DeepLX-like configuration in the plugin README

Motivation

The current Google backend relies on an unofficial endpoint and may fail due to
rate limiting or anti-abuse checks.

This change adds support for self-hosted DeepLX-like endpoints while keeping the
implementation small and preserving the existing Google and DeepL backends.

Notes

• existing google and deepl backends are unchanged
• for DeepLX instances using query-token auth, the token can be included directly
  in the configured API URL

[github-actions]

@Cleboost - this PR modifies your plugin. Please review when you have a chance.

[github-actions]

Automatic Code Quality Review

---

File: translator/LauncherProvider.qml

• (L) Line 145: When it comes to translations there is no need for fallback values. From: pluginApi?.tr("example") || "value". To: pluginApi?.tr("example")

+        return fallback || (pluginApi?.tr("messages.connectionError") || "Connection error");

• (L) Line 276: When it comes to translations there is no need for fallback values. From: pluginApi?.tr("example") || "value". To: pluginApi?.tr("example")

+                            callback(null, pluginApi?.tr("messages.error") || "Translation error");

• (L) Line 279: When it comes to translations there is no need for fallback values. From: pluginApi?.tr("example") || "value". To: pluginApi?.tr("example")

+                        callback(null, pluginApi?.tr("messages.error") || "Translation error");

---

File: translator/Settings.qml

• (H) Line 57: Use translations instead of hardcoded text. Instead of: "Example Label". To: pluginApi?.tr("panel.example-label")

+        label: "API URL"

• (L) Line 66: When it comes to translations there is no need for fallback values. From: pluginApi?.tr("example") || "value". To: pluginApi?.tr("example")

+        label: pluginApi?.tr("settings.apiKey-label") || "API Key"

[github-actions]

Automatic Code Quality Review

---

File: translator/LauncherProvider.qml

• (L) Line 235: When it comes to translations there is no need for fallback values. From: pluginApi?.tr("example") || "value". To: pluginApi?.tr("example")

+                    callback(null, pluginApi?.tr("messages.invalidApiKey") || "Invalid API Key");

[RanderDouble]

Addressed the i18n review feedback.

Changes in this update:

• removed the new hardcoded API URL label
• removed the new translation fallbacks introduced in the DeepLX-like backend
  changes
• kept the error handling scoped to the existing generic messages plus
  missingApiUrl
• added localized strings for the new DeepLX-like settings fields and missing URL
  message

[Cleboost]

thanks <3

[spiros132]

One minor feedback otherwise it lgtm :)

[spiros132]

I'm a bit confused why this is needed.