feat: enhance keyboard shortcuts and background image handling

- Added `GlobalKeyboardShortcuts` component to manage global keyboard shortcuts.
- Introduced new keyboard shortcut (Shift + B) for changing the background.
- Updated `BackgroundImageSelector` to preserve custom background URLs and allow restoration of previously set backgrounds.
- Improved local storage handling for custom backgrounds to enhance user experience.

src/contexts/KeyboardShortcutsContext.tsx

@@ -32,6 +32,11 @@ const PAGE_SHORTCUTS: PageShortcuts = {
       description: 'Faire tourner les thèmes dark',
       category: 'Apparence'
     },
+    {
+      keys: ['Shift', 'B'],
+      description: 'Changer le background',
+      category: 'Apparence'
+    },
     {
       keys: ['Esc'],
       description: 'Fermer les modales/annuler',