feat: implement drag & drop functionality using @dnd-kit

- Added drag & drop capabilities to the Kanban board with @dnd-kit for task status updates.
- Integrated DndContext in `KanbanBoard` and utilized `useDroppable` in `KanbanColumn` for drop zones.
- Enhanced `TaskCard` with draggable features and visual feedback during dragging.
- Updated `TODO.md` to reflect the completion of drag & drop tasks and optimistically update task statuses.
- Introduced optimistic updates in `useTasks` for smoother user experience during drag & drop operations.

components/ui/Header.tsx

@@ -10,22 +10,27 @@ interface HeaderProps {
     todo: number;
     completionRate: number;
   };
+  syncing?: boolean;
 }
 
-export function Header({ title, subtitle, stats }: HeaderProps) {
+export function Header({ title, subtitle, stats, syncing = false }: HeaderProps) {
   return (
     <header className="bg-slate-900/80 backdrop-blur-sm border-b border-slate-700/50 shadow-lg shadow-slate-900/20">
       <div className="container mx-auto px-6 py-4">
         <div className="flex flex-col sm:flex-row sm:items-center sm:justify-between gap-6">
           {/* Titre tech avec glow */}
           <div className="flex items-center gap-4">
-            <div className="w-3 h-3 bg-cyan-400 rounded-full animate-pulse shadow-cyan-400/50 shadow-lg"></div>
+            <div className={`w-3 h-3 rounded-full shadow-lg ${
+              syncing 
+                ? 'bg-yellow-400 animate-spin shadow-yellow-400/50' 
+                : 'bg-cyan-400 animate-pulse shadow-cyan-400/50'
+            }`}></div>
             <div>
               <h1 className="text-2xl font-mono font-bold text-slate-100 tracking-wider">
                 {title}
               </h1>
               <p className="text-slate-400 mt-1 font-mono text-sm">
-                {subtitle}
+                {subtitle} {syncing && '• Synchronisation...'}
               </p>
             </div>
           </div>