feat: integrate ToastProvider and enhance theme management

- Added `ToastProvider` to `RootLayout` for improved user feedback on theme changes.
- Updated `ThemeProvider` to display toast notifications with theme names and icons upon theme changes.
- Refactored theme-related imports to streamline code and improve maintainability.
- Simplified background cycling logic in `useBackgroundCycle` to utilize centralized background definitions.
- Cleaned up unused background definitions in `BackgroundContext` for better clarity and performance.

src/lib/theme-config.ts

@@ -40,7 +40,7 @@ export const THEME_CONFIG = {
     'solarized'
   ] as Theme[],
   
-  // Métadonnées des thèmes (pour l'UI future)
+  // Métadonnées des thèmes (déplacées vers ui-config.ts)
   metadata: {
     light: { name: 'Light', description: 'Thème clair par défaut', icon: '☀️' },
     dark: { name: 'Dark', description: 'Thème sombre classique', icon: '🌙' },