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/CouchbaseLiteAppSettingsService.cs

@@ -47,6 +47,7 @@ public sealed class CouchbaseLiteAppSettingsService : IAppSettingsService
     private void SaveDocument(AppSettingsDocument settings)
     {
         var document = new MutableDocument(DefaultSettingsId);
+        document.SetString("themeMode", settings.ThemeMode.ToString());
         document.SetDouble("standardWorkHoursPerDay", Decimal.ToDouble(settings.StandardWorkHoursPerDay));
         document.SetDouble("hourlyGrossRate", Decimal.ToDouble(settings.HourlyGrossRate));
         document.SetDouble("profitabilityCoefficient", Decimal.ToDouble(settings.ProfitabilityCoefficient));
@@ -65,6 +66,7 @@ public sealed class CouchbaseLiteAppSettingsService : IAppSettingsService
         return new AppSettingsDocument
         {
             Id = document.Id,
+            ThemeMode = ReadThemeMode(document),
             StandardWorkHoursPerDay = ReadDecimal(document, "standardWorkHoursPerDay", 8m),
             HourlyGrossRate = ReadDecimal(document, "hourlyGrossRate", 17.5m),
             ProfitabilityCoefficient = ReadDecimal(document, "profitabilityCoefficient", 0.67m),
@@ -84,6 +86,14 @@ public sealed class CouchbaseLiteAppSettingsService : IAppSettingsService
             : defaultValue;
     }
 
+    private static AppThemeMode ReadThemeMode(Document document)
+    {
+        var value = document.GetString("themeMode");
+        return Enum.TryParse<AppThemeMode>(value, ignoreCase: true, out var themeMode)
+            ? themeMode
+            : AppThemeMode.System;
+    }
+
     private static DateTimeOffset ReadDateTimeOffset(Document document, string key)
     {
         var value = document.GetString(key);