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_de.arb

@@ -260,6 +260,10 @@
   "followingSystem": "Dem System folgen: {theme}",
   "@followingSystem": {"placeholders": {"theme": {"type": "String"}}},
   "themeDark": "Dunkel",
+  "themePalette": "Farbpalette",
+  "@themePalette": {"description": "Titel für die Auswahl der App-Farbpalette."},
+  "themePaletteDescription": "Wählen Sie die Akzentfarben für Schaltflächen, Karten und Chatblasen.",
+  "@themePaletteDescription": {"description": "Hilfetext zur Erklärung der Palettenauswahl."},
   "themeLight": "Hell",
   "currentlyUsingDarkTheme": "Aktuell dunkles Thema",
   "currentlyUsingLightTheme": "Aktuell helles Thema",