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.

Components/Pages/Settings.razor

@@ -2,6 +2,7 @@
 @attribute [Authorize]
 
 @inject IAppSettingsService AppSettingsService
+@inject AppThemeState ThemeState
 
 <PageTitle>Settings</PageTitle>
 
@@ -18,6 +19,14 @@ else
         <DataAnnotationsValidator />
 
         <div class="row g-3">
+            <div class="col-12 col-md-6">
+                <label class="form-label">Theme</label>
+                <InputSelect class="form-select" @bind-Value="settings.ThemeMode">
+                    <option value="@AppThemeMode.System">Follow system</option>
+                    <option value="@AppThemeMode.Light">Light</option>
+                    <option value="@AppThemeMode.Dark">Dark</option>
+                </InputSelect>
+            </div>
             <div class="col-12 col-md-6">
                 <label class="form-label">Standard work hours/day</label>
                 <InputNumber class="form-control" @bind-Value="settings.StandardWorkHoursPerDay" />
@@ -75,6 +84,7 @@ else
         }
 
         settings = await AppSettingsService.SaveAsync(settings);
+        ThemeState.SetThemeMode(settings.ThemeMode);
         statusMessage = $"Saved at {DateTime.Now:t}";
     }
 }