chore: release v2.4

- Add theme system (Dark/Light palettes, ModernTheme, ThemeManager)
- Add InputDialog, Spinner common view
- Add DuplicatesCsvExportService
- Refresh views, dialogs, and view models across tabs
- Update localization strings (en/fr)
- Tweak services (transfer, permissions, search, user access, ownership elevation, bulk operations)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

SharepointToolbox/Services/SettingsService.cs

@@ -43,4 +43,14 @@ public class SettingsService
         settings.AutoTakeOwnership = enabled;
         await _repository.SaveAsync(settings);
     }
+
+    public async Task SetThemeAsync(string mode)
+    {
+        if (mode is not ("System" or "Light" or "Dark"))
+            throw new ArgumentException($"Unsupported theme '{mode}'. Supported: System, Light, Dark.", nameof(mode));
+
+        var settings = await _repository.LoadAsync();
+        settings.Theme = mode;
+        await _repository.SaveAsync(settings);
+    }
 }