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/ThemeToggle.tsx

@@ -0,0 +1,51 @@
+/**
+ * Theme Toggle Button
+ * Switches between light, dark, and system themes
+ */
+
+'use client';
+
+import { Moon, Sun, Monitor } from 'lucide-react';
+import { Button } from '@/components/ui/button';
+import {
+  DropdownMenu,
+  DropdownMenuContent,
+  DropdownMenuItem,
+  DropdownMenuTrigger,
+} from '@/components/ui/dropdown-menu';
+import { useTheme } from './ThemeProvider';
+
+export function ThemeToggle() {
+  const { theme, setTheme, resolvedTheme } = useTheme();
+
+  return (
+    <DropdownMenu>
+      <DropdownMenuTrigger asChild>
+        <Button variant="ghost" size="icon" aria-label="Toggle theme">
+          {resolvedTheme === 'dark' ? (
+            <Moon className="h-5 w-5" />
+          ) : (
+            <Sun className="h-5 w-5" />
+          )}
+        </Button>
+      </DropdownMenuTrigger>
+      <DropdownMenuContent align="end">
+        <DropdownMenuItem onClick={() => setTheme('light')}>
+          <Sun className="mr-2 h-4 w-4" />
+          Light
+          {theme === 'light' && <span className="ml-auto">✓</span>}
+        </DropdownMenuItem>
+        <DropdownMenuItem onClick={() => setTheme('dark')}>
+          <Moon className="mr-2 h-4 w-4" />
+          Dark
+          {theme === 'dark' && <span className="ml-auto">✓</span>}
+        </DropdownMenuItem>
+        <DropdownMenuItem onClick={() => setTheme('system')}>
+          <Monitor className="mr-2 h-4 w-4" />
+          System
+          {theme === 'system' && <span className="ml-auto">✓</span>}
+        </DropdownMenuItem>
+      </DropdownMenuContent>
+    </DropdownMenu>
+  );
+}