Add dark mode with system preference auto-detection

All user-facing components now support dark mode via prefers-color-scheme.
- Dark backgrounds: gray-950/900/800
- Dark text: gray-100/200/300/400
- Orange brand colors adapt with darker tints
- Glass effects work in both modes
- Skeletons, cards, filters, bottom sheet all themed
- Google Maps InfoWindow stays light (maps don't support dark)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/SearchBar.tsx

@@ -25,12 +25,12 @@ export default function SearchBar({ onSearch, isLoading }: SearchBarProps) {
         value={query}
         onChange={(e) => setQuery(e.target.value)}
         placeholder="식당, 지역, 음식..."
-        className="flex-1 min-w-0 px-3 py-2 border border-gray-300 rounded-lg focus:outline-none focus:ring-2 focus:ring-orange-400 text-sm"
+        className="flex-1 min-w-0 px-3 py-2 border border-gray-300 dark:border-gray-700 rounded-lg focus:outline-none focus:ring-2 focus:ring-orange-400 text-sm bg-white dark:bg-gray-800 dark:text-gray-200 dark:placeholder-gray-500"
       />
       <select
         value={mode}
         onChange={(e) => setMode(e.target.value as typeof mode)}
-        className="shrink-0 px-2 py-2 border border-gray-300 rounded-lg text-sm bg-white"
+        className="shrink-0 px-2 py-2 border border-gray-300 dark:border-gray-700 rounded-lg text-sm bg-white dark:bg-gray-800 dark:text-gray-300"
       >
         <option value="hybrid">통합</option>
         <option value="keyword">키워드</option>