TP-27: Startup fallback for missing/broken selected theme

lib/core/theme/theme_controller.dart

@@ -20,6 +20,10 @@ class ThemeController extends ChangeNotifier {
   static const String builtinQueryaDarkId = 'querya-dark';
   static const String builtinQueryaLightId = 'querya-light';
 
+  /// Shown in Preferences when persisted registry selection cannot be restored.
+  static const String selectedThemeStartupFallbackMessage =
+      'Selected theme failed to load. Using Querya Dark.';
+
   static const ThemeDefinition builtinQueryaDarkDefinition = ThemeDefinition(
     id: builtinQueryaDarkId,
     name: 'Querya Dark',
@@ -245,16 +249,27 @@ class ThemeController extends ChangeNotifier {
       path: _selectedThemePath,
     );
     if (stillAvailable == null) {
-      _selectedThemeLoadError =
-          'Selected theme "$selectedId" is not available.';
+      if (_registryTheme != null) {
+        // Keep the in-memory active theme; only the on-disk scan lost the file.
+        _selectedThemeLoadError = selectedThemeStartupFallbackMessage;
+        return;
+      }
+      _markRegistrySelectionFailed();
       return;
     }
 
     if (_registryTheme != null) {
       _selectedThemeLoadError = null;
+      _registrySelectionFailed = false;
     }
   }
 
+  void _markRegistrySelectionFailed() {
+    _registryTheme = null;
+    _registrySelectionFailed = true;
+    _selectedThemeLoadError = selectedThemeStartupFallbackMessage;
+  }
+
   Future<void> setThemeById(String id) async {
     if (id == builtinQueryaDarkId) {
       await _applyBuiltinPreset(QueryaThemePreset.queryaDark);
@@ -486,9 +501,7 @@ class ThemeController extends ChangeNotifier {
       path: _selectedThemePath,
     );
     if (definition == null) {
-      _registrySelectionFailed = true;
-      _selectedThemeLoadError =
-          'Selected theme "${_selectedThemeId!}" is not available.';
+      _markRegistrySelectionFailed();
       return;
     }
 
@@ -498,12 +511,13 @@ class ThemeController extends ChangeNotifier {
         _registryTheme = theme;
         _selectedThemeId = definition.id;
         _selectedThemePath = definition.path;
+        _registrySelectionFailed = false;
+        _selectedThemeLoadError = null;
         _themeMode = theme.brightness == Brightness.light
             ? ThemeMode.light
             : ThemeMode.dark;
-      case ThemeLoadFailure(:final message):
-        _registrySelectionFailed = true;
-        _selectedThemeLoadError = message;
+      case ThemeLoadFailure():
+        _markRegistrySelectionFailed();
     }
   }
 

test/core/theme/theme_controller_test.dart

@@ -244,14 +244,73 @@ void main() {
       await c.load();
 
       expect(c.activeTheme, QueryaTheme.darkDefault);
-      expect(c.selectedThemeLoadError, isNotNull);
+      expect(
+        c.selectedThemeLoadError,
+        ThemeController.selectedThemeStartupFallbackMessage,
+      );
       expect(await AppSettings.instance.getSelectedThemeId(), 'missing-theme');
       expect(
         await AppSettings.instance.getThemePreset(),
         QueryaThemePreset.queryaLight,
       );
     });
 
+    test('missing theme file on startup falls back to Querya Dark', () async {
+      final c = ThemeController.instance;
+      final themeFile = File(p.join(themesDir.path, 'querya_custom_dark.json'));
+      await _copyFixture('querya_custom_dark.json', themeFile);
+      await c.load();
+      await c.setThemeById('fixture-custom-dark');
+      await themeFile.delete();
+
+      await c.load();
+
+      expect(c.activeTheme, QueryaTheme.darkDefault);
+      expect(
+        c.selectedThemeLoadError,
+        ThemeController.selectedThemeStartupFallbackMessage,
+      );
+      expect(c.selectedThemeId, 'fixture-custom-dark');
+      expect(await AppSettings.instance.getSelectedThemeId(), 'fixture-custom-dark');
+    });
+
+    test('invalid theme file skipped on startup falls back to Querya Dark',
+        () async {
+      final c = ThemeController.instance;
+      final themeFile = File(p.join(themesDir.path, 'broken-theme.json'));
+      await _copyFixture('querya_custom_invalid_missing_id.json', themeFile);
+      await AppSettings.instance.setSelectedThemeId('broken-theme');
+      await AppSettings.instance.setSelectedThemeSource('filesystem');
+      await AppSettings.instance.setSelectedThemePath(themeFile.path);
+
+      await c.load();
+
+      expect(c.activeTheme, QueryaTheme.darkDefault);
+      expect(
+        c.selectedThemeLoadError,
+        ThemeController.selectedThemeStartupFallbackMessage,
+      );
+      expect(await AppSettings.instance.getSelectedThemeId(), 'broken-theme');
+    });
+
+    test('valid theme selection after startup failure clears error', () async {
+      final c = ThemeController.instance;
+      await AppSettings.instance.setSelectedThemeId('missing-theme');
+      await AppSettings.instance.setSelectedThemeSource('filesystem');
+      await c.load();
+      expect(c.selectedThemeLoadError, isNotNull);
+
+      await _copyFixture(
+        'querya_custom_dark.json',
+        File(p.join(themesDir.path, 'querya_custom_dark.json')),
+      );
+      await c.loadAvailableThemes();
+      await c.setThemeById('fixture-custom-dark');
+
+      expect(c.selectedThemeLoadError, isNull);
+      expect(c.activeTheme.colorScheme.primary, parseQueryaThemeColor('#38BDF8'));
+    });
+
     test('setPreset clears registry selection', () async {
       final c = ThemeController.instance;
       await _copyFixture(