feat: implement dynamic theme palette selection

- Introduced a new feature allowing users to select from multiple accent color palettes for buttons, cards, and chat bubbles.
- Added `AppThemePalette` provider to manage the current theme palette and persist user selections.
- Updated the `AppTheme` class to utilize the selected palette for light and dark themes, enhancing visual customization.
- Enhanced the `AppCustomizationPage` to include a palette selector, improving user experience and personalization options.
- Updated localization files to support new palette selection UI elements in multiple languages.

lib/l10n/app_localizations_en.dart

@@ -751,6 +751,13 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get themeDark => 'Dark';
 
+  @override
+  String get themePalette => 'Accent palette';
+
+  @override
+  String get themePaletteDescription =>
+      'Choose the accent colors used for buttons, cards, and chat bubbles.';
+
   @override
   String get themeLight => 'Light';