feat: enhance Header component with active link styling

- Added `usePathname` hook to determine the current route.
- Implemented `isActiveLink` and `getLinkClasses` functions for dynamic link styling based on active state.
- Adjusted navigation links in the Header to utilize the new styling functions for improved user experience.
- Updated KanbanPageClient subtitle for consistency.

src/app/kanban/KanbanPageClient.tsx

@@ -57,7 +57,7 @@ function KanbanPageContent() {
     <div className="min-h-screen bg-[var(--background)]">
       <Header 
         title="Kanban Board" 
-        subtitle="Gestionnaire de tâches en colonnes"
+        subtitle="Gestionnaire de tâches"
         stats={stats}
         syncing={syncing}
       />