fix(sync): address PR review round 2

- Rebuild flow now reflects the actual outcome: on failure (e.g. no epoch
  marker) it shows the error message instead of a misleading success
  snackbar.
- resolveSyncTempDir's FlutterError fallback is scoped to the specific
  'Binding has not yet been initialized' message; any other FlutterError
  rethrows so real problems surface.
- deleteLeftoverBaseTempFiles sweeps only the 'ssv1_' sync prefix (covers
  base export + assembled parts) instead of any '*.base' file, so unrelated
  temp files in the shared dir are never deleted.

Author: ericgriffin

lib/core/services/sync/changeset_log/sync_temp_dir.dart

@@ -25,7 +25,14 @@ Future<Directory> resolveSyncTempDir() async {
     return await getTemporaryDirectory();
   } on MissingPluginException {
     return Directory.systemTemp;
-  } on FlutterError {
-    return Directory.systemTemp;
+  } on FlutterError catch (e) {
+    // ONLY the "no test binding" case (a unit test that never called
+    // TestWidgetsFlutterBinding.ensureInitialized, so ServicesBinding.instance
+    // throws). Any other FlutterError is a real problem and must surface, not
+    // silently fall back to /tmp.
+    if (e.toString().contains('Binding has not yet been initialized')) {
+      return Directory.systemTemp;
+    }
+    rethrow;
   }
 }

lib/core/services/sync/sync_service.dart

@@ -2072,15 +2072,18 @@ class SyncService {
   }
 
   /// Best-effort sweep of leftover streaming-base temp files (base export
-  /// `ssv1_base_*` and assembled `*.base` parts) from the app temp dir. Failure
-  /// is logged and ignored -- a stale temp file is harmless.
+  /// base export `ssv1_base_*.json` and assembled `ssv1_*.base` / `ssv1_adopt_*`
+  /// parts) from the app temp dir. Every sync temp file is prefixed `ssv1_`, so
+  /// the sweep matches ONLY that prefix -- never an unrelated app temp file
+  /// (the dir is a shared, general-purpose temp location). Failure is logged
+  /// and ignored; a stale temp file is harmless.
   Future<void> deleteLeftoverBaseTempFiles() async {
     try {
       final dir = await resolveSyncTempDir();
       await for (final entity in dir.list(followLinks: false)) {
         if (entity is! File) continue;
         final name = entity.uri.pathSegments.last;
-        if (name.startsWith('ssv1_base_') || name.endsWith('.base')) {
+        if (name.startsWith('ssv1_')) {
           try {
             await entity.delete();
           } catch (_) {

lib/features/settings/presentation/pages/troubleshoot_sync_page.dart

@@ -119,11 +119,21 @@ class TroubleshootSyncPage extends ConsumerWidget {
     );
     if (ok != true) return;
     await ref.read(syncStateProvider.notifier).rebuildBackendFromThisDevice();
-    if (context.mounted) {
-      ScaffoldMessenger.of(context).showSnackBar(
-        const SnackBar(content: Text('Rebuilt backend from this device')),
-      );
-    }
+    if (!context.mounted) return;
+    // The rebuild can fail (e.g. no epoch marker to rebuild from), in which case
+    // the notifier surfaces SyncStatus.error -- reflect that instead of always
+    // claiming success.
+    final state = ref.read(syncStateProvider);
+    final failed = state.status == SyncStatus.error;
+    ScaffoldMessenger.of(context).showSnackBar(
+      SnackBar(
+        content: Text(
+          failed
+              ? (state.message ?? 'Rebuild failed')
+              : 'Rebuilt backend from this device',
+        ),
+      ),
+    );
   }
 
   Future<void> _confirmRemoveThisDevice(

test/core/services/sync/sync_service_repair_test.dart

@@ -70,12 +70,16 @@ void main() {
       await epochStore.setPendingReplace(marker);
       final leftover = File('${fakeAppTemp.path}/ssv1_base_dev_0.abc.json');
       await leftover.writeAsString('stale');
+      // An unrelated temp file (the dir is shared) must be left untouched.
+      final unrelated = File('${fakeAppTemp.path}/user_photo.jpg');
+      await unrelated.writeAsString('keep');
 
       await buildService().repairLocalSyncState();
 
       expect(epochStore.lastAcceptedMarker, isNull);
       expect(epochStore.pendingReplace, isNull);
       expect(leftover.existsSync(), isFalse);
+      expect(unrelated.existsSync(), isTrue);
     },
   );
 

test/features/settings/presentation/pages/troubleshoot_sync_page_test.dart

@@ -17,6 +17,10 @@ class _FakeSyncNotifier extends StateNotifier<SyncState>
   int wipeAllCalls = 0;
   int rebuildCalls = 0;
 
+  /// When set, [rebuildBackendFromThisDevice] models a failure by leaving the
+  /// state in [SyncStatus.error] with this message (mirrors the real notifier).
+  String? rebuildFailureMessage;
+
   @override
   Future<void> repairSync() async => repairSyncCalls++;
 
@@ -27,7 +31,13 @@ class _FakeSyncNotifier extends StateNotifier<SyncState>
   Future<void> wipeAllCloudSyncData() async => wipeAllCalls++;
 
   @override
-  Future<void> rebuildBackendFromThisDevice() async => rebuildCalls++;
+  Future<void> rebuildBackendFromThisDevice() async {
+    rebuildCalls++;
+    final msg = rebuildFailureMessage;
+    if (msg != null) {
+      state = SyncState(status: SyncStatus.error, message: msg);
+    }
+  }
 
   @override
   dynamic noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
@@ -155,6 +165,21 @@ void main() {
     expect(find.text('Rebuilt backend from this device'), findsOneWidget);
   });
 
+  testWidgets('Rebuild failure shows the error, not a success snackbar', (
+    tester,
+  ) async {
+    final fake = await _pump(tester);
+    fake.rebuildFailureMessage = 'No library replacement to rebuild from';
+
+    await tester.tap(find.text('Rebuild backend from this device'));
+    await tester.pumpAndSettle();
+    await tester.tap(find.widgetWithText(FilledButton, 'Rebuild'));
+    await tester.pumpAndSettle();
+
+    expect(find.text('No library replacement to rebuild from'), findsOneWidget);
+    expect(find.text('Rebuilt backend from this device'), findsNothing);
+  });
+
   testWidgets('Remove-this-device confirm calls the notifier', (tester) async {
     final fake = await _pump(tester);