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/core/persistence/persistence_keys.dart

@@ -18,6 +18,7 @@ final class PreferenceKeys {
   static const String rememberCredentials = 'remember_credentials';
   static const String activeServerId = 'active_server_id';
   static const String themeMode = 'theme_mode';
+  static const String themePalette = 'theme_palette_v1';
   static const String localeCode = 'locale_code_v1';
   static const String onboardingSeen = 'onboarding_seen_v1';
   static const String reviewerMode = 'reviewer_mode_v1';

lib/core/persistence/persistence_migrator.dart

@@ -89,6 +89,7 @@ class PersistenceMigrator {
     copyBool(PreferenceKeys.rememberCredentials);
     copyString(PreferenceKeys.activeServerId);
     copyString(PreferenceKeys.themeMode);
+    copyString(PreferenceKeys.themePalette);
     copyString(PreferenceKeys.localeCode);
     copyBool(PreferenceKeys.onboardingSeen);
     copyBool(PreferenceKeys.reviewerMode);
@@ -194,6 +195,7 @@ class PersistenceMigrator {
       PreferenceKeys.rememberCredentials,
       PreferenceKeys.activeServerId,
       PreferenceKeys.themeMode,
+      PreferenceKeys.themePalette,
       PreferenceKeys.localeCode,
       PreferenceKeys.onboardingSeen,
       PreferenceKeys.reviewerMode,