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/Core/Models/AppSettings.cs

@@ -5,4 +5,5 @@ public class AppSettings
     public string DataFolder { get; set; } = string.Empty;
     public string Lang { get; set; } = "en";
     public bool AutoTakeOwnership { get; set; } = false;
+    public string Theme { get; set; } = "System"; // System | Light | Dark
 }