feat: add dark/light/system theme toggle

Implemented complete theme switching with three modes:
- Light theme
- Dark theme
- System (auto-detect from OS)

Changes:
- Created ModeToggle component with dropdown menu (src/components/mode-toggle.tsx)
- Added theme toggle to desktop header (Index.tsx)
- Enhanced mobile settings with radio group for theme selection (SettingsPage.tsx)
- Changed default theme from "light" to "system" for better UX (App.tsx)

Features:
✅ Seamless theme switching without page reload
✅ Persists user preference in localStorage
✅ System theme auto-detects OS preference
✅ Smooth transitions between themes
✅ Works on both mobile and desktop

Desktop: Theme toggle button in top navigation
Mobile: Theme settings in Settings page with Light/Dark/System options

src/pages/Index.tsx

@@ -32,6 +32,7 @@ import { ConfirmModal } from "@/components/modals/ConfirmModal";
 import { InputModal } from "@/components/modals/InputModal";
 import { AddUserModal } from "@/components/modals/AddUserModal";
 import { compressImage, formatFileSize } from "@/utils/imageCompression";
+import { ModeToggle } from "@/components/mode-toggle";
 
 // ===== FONCTIONS UTILITAIRES TIMEZONE EUROPE/PARIS =====
 
@@ -923,6 +924,8 @@ const Index = () => {
                 Nouvelle Note
               </Button>
 
+              <ModeToggle />
+
               {user && (
                 <DropdownMenu>
                   <DropdownMenuTrigger asChild>