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/App.tsx

@@ -75,7 +75,7 @@ const App = () => {
 
   return (
     <QueryClientProvider client={queryClient}>
-      <ThemeProvider defaultTheme="light" storageKey="notes-theme">
+      <ThemeProvider defaultTheme="system" storageKey="notes-theme">
         <TooltipProvider>
           <BrowserRouter>
             <Routes>