feat(frontend): implement basic UI foundation (Issue #6)

frontend/src/components/ThemeToggle.tsx

@@ -0,0 +1,12 @@
+import { useTheme } from '../context/ThemeContext'
+import { Button } from './ui/Button'
+
+export function ThemeToggle() {
+  const { theme, toggleTheme } = useTheme()
+
+  return (
+    <Button variant="ghost" size="sm" onClick={toggleTheme} title={`Switch to ${theme === 'dark' ? 'light' : 'dark'} mode`}>
+      {theme === 'dark' ? '☀️' : '🌙'}
+    </Button>
+  )
+}