refactor: update theme management and enhance UI components

- Refactored theme imports in `preferences.ts` and `ThemeSelector.tsx` to use centralized `theme-config`.
- Added new CSS variables for special cards in `globals.css` to improve theme consistency.
- Enhanced `Header` and `TaskCard` components with theme dropdown functionality for better user experience.
- Updated `ThemeProvider` to support cycling through dark themes, improving theme selection flexibility.
- Cleaned up unused imports and streamlined component structures for better maintainability.

src/components/KeyboardShortcuts.tsx

@@ -0,0 +1,8 @@
+'use client';
+
+import { useKeyboardShortcuts } from '@/hooks/useKeyboardShortcuts';
+
+export function KeyboardShortcuts() {
+  useKeyboardShortcuts();
+  return null; // Ce composant ne rend rien, il gère juste les raccourcis
+}