Add light/dark mode theme support with OS sync

- Added theme state management (light/dark/system)
- Implemented OS theme detection and automatic sync
- Added theme toggle UI in sidebar with 3 buttons
- Applied dark mode styles to all components:
  - Sidebar with dark backgrounds and borders
  - Note editor with dark text and inputs
  - Toolbar buttons with dark hover states
  - TipTap editor with dark mode text and code blocks
- Theme preference saved to localStorage
- Enabled Tailwind dark mode with class strategy
- Smooth transitions between themes

tailwind.config.js

@@ -1,5 +1,6 @@
 /** @type {import('tailwindcss').Config} */
 export default {
+  darkMode: 'class',
   content: [
     "./index.html",
     "./src/**/*.{js,ts,jsx,tsx}",