feat: implement theme system and UI updates

- Added theme context and provider for light/dark mode support.
- Integrated theme toggle button in the Header component.
- Updated UI components to utilize CSS variables for consistent theming.
- Enhanced Kanban components and forms with new theme styles for better visual coherence.
- Adjusted global styles to define color variables for both themes, improving maintainability.

components/kanban/Board.tsx

@@ -110,12 +110,12 @@ export function KanbanBoard({ tasks, onCreateTask, onDeleteTask, onEditTask, onU
       onDragStart={handleDragStart}
       onDragEnd={handleDragEnd}
     >
-      <div className="h-full flex flex-col bg-slate-950">
+      <div className="h-full flex flex-col bg-[var(--background)]">
         {/* Header avec bouton nouvelle tâche */}
         <div className="flex justify-between items-center p-6 pb-0">
           <div className="flex items-center gap-3">
-            <div className="w-2 h-2 bg-cyan-400 rounded-full animate-pulse"></div>
-            <h2 className="text-lg font-mono font-bold text-slate-100 uppercase tracking-wider">
+            <div className="w-2 h-2 bg-[var(--primary)] rounded-full animate-pulse"></div>
+            <h2 className="text-lg font-mono font-bold text-[var(--foreground)] uppercase tracking-wider">
               Kanban Board
             </h2>
           </div>