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/components/mode-toggle.tsx

@@ -0,0 +1,36 @@
+import { Moon, Sun } from "lucide-react";
+import { Button } from "@/components/ui/button";
+import {
+  DropdownMenu,
+  DropdownMenuContent,
+  DropdownMenuItem,
+  DropdownMenuTrigger,
+} from "@/components/ui/dropdown-menu";
+import { useTheme } from "@/components/theme-provider";
+
+export function ModeToggle() {
+  const { setTheme } = useTheme();
+
+  return (
+    <DropdownMenu>
+      <DropdownMenuTrigger asChild>
+        <Button variant="outline" size="icon">
+          <Sun className="h-[1.2rem] w-[1.2rem] rotate-0 scale-100 transition-all dark:-rotate-90 dark:scale-0" />
+          <Moon className="absolute h-[1.2rem] w-[1.2rem] rotate-90 scale-0 transition-all dark:rotate-0 dark:scale-100" />
+          <span className="sr-only">Toggle theme</span>
+        </Button>
+      </DropdownMenuTrigger>
+      <DropdownMenuContent align="end">
+        <DropdownMenuItem onClick={() => setTheme("light")}>
+          Clair
+        </DropdownMenuItem>
+        <DropdownMenuItem onClick={() => setTheme("dark")}>
+          Sombre
+        </DropdownMenuItem>
+        <DropdownMenuItem onClick={() => setTheme("system")}>
+          Système
+        </DropdownMenuItem>
+      </DropdownMenuContent>
+    </DropdownMenu>
+  );
+}