feat: add theme mode support with AppThemeMode enum and AppThemeState service

- Introduced AppThemeMode enum to define theme options: System, Light, Dark.
- Updated AppSettingsDocument to include ThemeMode property.
- Created AppThemeState service to manage current theme mode and handle changes.
- Integrated theme mode handling in CouchbaseLiteAppSettingsService for persistence.
- Added JavaScript for theme management in the frontend, supporting system preference detection.
- Enhanced CSS with theme variables for consistent styling across light and dark modes.
- Updated Playwright tests to ensure sidebar functionality and responsiveness.

Domain/AppSettingsDocument.cs

@@ -4,6 +4,8 @@ public sealed class AppSettingsDocument
 {
     public string Id { get; set; } = "global";
 
+    public AppThemeMode ThemeMode { get; set; } = AppThemeMode.System;
+
     public decimal StandardWorkHoursPerDay { get; set; } = 8m;
 
     public decimal HourlyGrossRate { get; set; } = 17.5m;