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/HomePageClient.tsx

@@ -15,7 +15,7 @@ function HomePageContent() {
   const { stats, syncing } = useTasksContext();
 
   return (
-    <div className="min-h-screen bg-slate-950">
+    <div className="min-h-screen bg-[var(--background)]">
       <Header 
         title="TowerControl" 
         subtitle="Gestionnaire de tâches moderne"