Add theme toggle with light, dark, and system support

- **Header:** Integrate `ThemeToggle` component into the user menu area.
- **Theme Provider:** Introduce `ThemeProvider` context for managing and persisting theme preferences.
- **New Components:** Add `ThemeToggle` for switching themes and `ThemeProvider` to handle state and system preferences.
- Ensure responsive updates and localStorage persistence for user-selected themes.

frontend/src/components/theme/index.ts

@@ -0,0 +1,6 @@
+/**
+ * Theme components
+ */
+
+export { ThemeProvider, useTheme } from './ThemeProvider';
+export { ThemeToggle } from './ThemeToggle';