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.

Services/Settings/AppThemeState.cs

@@ -0,0 +1,41 @@
+using WorkTracker.Domain;
+
+namespace WorkTracker.Services.Settings;
+
+public sealed class AppThemeState
+{
+    private readonly IAppSettingsService appSettingsService;
+    private AppThemeMode? currentThemeMode;
+
+    public AppThemeState(IAppSettingsService appSettingsService)
+    {
+        this.appSettingsService = appSettingsService;
+    }
+
+    public event Action<AppThemeMode>? ThemeModeChanged;
+
+    public AppThemeMode CurrentThemeMode => currentThemeMode ?? AppThemeMode.System;
+
+    public async Task<AppThemeMode> EnsureLoadedAsync(CancellationToken cancellationToken = default)
+    {
+        if (currentThemeMode.HasValue)
+        {
+            return currentThemeMode.Value;
+        }
+
+        var settings = await appSettingsService.GetAsync(cancellationToken);
+        currentThemeMode = settings.ThemeMode;
+        return CurrentThemeMode;
+    }
+
+    public void SetThemeMode(AppThemeMode themeMode)
+    {
+        if (currentThemeMode == themeMode)
+        {
+            return;
+        }
+
+        currentThemeMode = themeMode;
+        ThemeModeChanged?.Invoke(themeMode);
+    }
+}