fix: fix light/dark/system theme switcher

- Add darkMode: class to tailwind config for class-based dark mode
- Load saved theme from localStorage on mount and apply immediately
- Add media query listener for system theme changes in system mode

frontend/tailwind.config.js

@@ -3,6 +3,7 @@ export default {
     "./index.html",
     "./src/**/*.{js,ts,jsx,tsx}",
   ],
+  darkMode: 'class',
   theme: {
     extend: {},
   },