feat: Implement dark mode support across the application

- Added ThemeContext to manage theme state and toggle functionality.
- Updated components to support dark mode styles, including header, dashboard, and home page.
- Enhanced CSS for smooth transitions between light and dark themes.
- Modified authentication context to handle async login operations.
- Improved user experience by preserving theme preference in local storage.
- Refactored login and register pages to handle OAuth tokens and errors more gracefully.

ospabhost/frontend/tailwind.config.js

@@ -4,6 +4,7 @@ export default {
     "./index.html",
     "./src/**/*.{js,ts,jsx,tsx}",
   ],
+  darkMode: 'class',
   theme: {
     extend: {
       colors: {