fix(a11y): #301+#302 모달 접근성 + race condition + 필터 상태 동기화

CRITICAL — 모달 접근성:
- frontend/src/lib/hooks/useModalA11y.ts 신규 (useEscapeKey, useFocusTrap, useBodyScrollLock)
- BottomSheet: role='dialog' / aria-modal / aria-label / ESC 닫기 / focus trap
- FilterSheet: role='dialog' / aria-modal / aria-labelledby / ESC 닫기 / focus trap, 닫기 버튼 aria-label

MAJOR — race condition (#301):
- RestaurantDetail useEffect에 cancelled 플래그 추가 → restaurant.id 변경 시 이전 fetch 결과 폐기

MAJOR — 필터 상태 동기화 (#302):
- page.tsx에 exitSearchMode 헬퍼 추가
- 검색 모드(isSearchResult=true)에서 cuisine/price/country/city/district 변경 시 자동으로 검색 모드 해제 + 원본 restaurants 재로드

후속 분리: #319(BottomSheet 매직넘버/UX), #320(필터 정밀도/접근성/테스트)

Refs: #301 #302

frontend/src/components/RestaurantDetail.tsx

@@ -23,19 +23,20 @@ export default function RestaurantDetail({
   const [favLoading, setFavLoading] = useState(false);
 
   useEffect(() => {
+    let cancelled = false;
     setLoading(true);
     api
       .getRestaurantVideos(restaurant.id)
-      .then(setVideos)
-      .catch(() => setVideos([]))
-      .finally(() => setLoading(false));
+      .then((v) => { if (!cancelled) setVideos(v); })
+      .catch(() => { if (!cancelled) setVideos([]); })
+      .finally(() => { if (!cancelled) setLoading(false); });
 
-    // Load favorite status if logged in
     if (getToken()) {
       api.getFavoriteStatus(restaurant.id)
-        .then((r) => setFavorited(r.favorited))
+        .then((r) => { if (!cancelled) setFavorited(r.favorited); })
         .catch(() => {});
     }
+    return () => { cancelled = true; };
   }, [restaurant.id]);
 
   const handleToggleFavorite = async () => {
@@ -57,12 +58,12 @@ export default function RestaurantDetail({
             <button
               onClick={handleToggleFavorite}
               disabled={favLoading}
-              className={`text-xl leading-none transition-colors ${
+              className={`p-1.5 -m-1.5 transition-colors touch-manipulation ${
                 favorited ? "text-rose-500" : "text-gray-300 dark:text-gray-600 hover:text-rose-400"
               }`}
               title={favorited ? "찜 해제" : "찜하기"}
             >
-              <Icon name="favorite" size={20} filled={favorited} />
+              <Icon name="favorite" size={22} filled={favorited} />
             </button>
           )}
           {restaurant.business_status === "CLOSED_PERMANENTLY" && (
@@ -218,7 +219,7 @@ export default function RestaurantDetail({
                   <Icon name="play_circle" size={16} filled className="flex-shrink-0" />
                   {v.title}
                 </a>
-                {v.foods_mentioned.length > 0 && (
+                {v.foods_mentioned?.length > 0 && (
                   <div className="flex flex-wrap gap-1 mt-2">
                     {v.foods_mentioned.map((f, i) => (
                       <span
@@ -235,7 +236,7 @@ export default function RestaurantDetail({
                     {v.evaluation.text}
                   </p>
                 )}
-                {v.guests.length > 0 && (
+                {v.guests?.length > 0 && (
                   <p className="mt-1 text-xs text-gray-500 dark:text-gray-400">
                     게스트: {v.guests.join(", ")}
                   </p>