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/lib/hooks/useModalA11y.ts

@@ -0,0 +1,74 @@
+import { useEffect } from "react";
+
+export function useEscapeKey(active: boolean, onEscape: () => void) {
+  useEffect(() => {
+    if (!active) return;
+    const handler = (e: KeyboardEvent) => {
+      if (e.key === "Escape") {
+        e.stopPropagation();
+        onEscape();
+      }
+    };
+    document.addEventListener("keydown", handler);
+    return () => document.removeEventListener("keydown", handler);
+  }, [active, onEscape]);
+}
+
+const FOCUSABLE = [
+  "a[href]",
+  "button:not([disabled])",
+  "input:not([disabled])",
+  "select:not([disabled])",
+  "textarea:not([disabled])",
+  '[tabindex]:not([tabindex="-1"])',
+].join(",");
+
+export function useFocusTrap<T extends HTMLElement>(active: boolean, containerRef: React.RefObject<T | null>) {
+  useEffect(() => {
+    if (!active) return;
+    const node = containerRef.current;
+    if (!node) return;
+    const previouslyFocused = document.activeElement as HTMLElement | null;
+    const focusables = () => Array.from(node.querySelectorAll<HTMLElement>(FOCUSABLE));
+    const first = focusables()[0];
+    first?.focus();
+    const handler = (e: KeyboardEvent) => {
+      if (e.key !== "Tab") return;
+      const list = focusables();
+      if (list.length === 0) {
+        e.preventDefault();
+        return;
+      }
+      const head = list[0];
+      const tail = list[list.length - 1];
+      const current = document.activeElement as HTMLElement | null;
+      if (e.shiftKey) {
+        if (current === head || !node.contains(current)) {
+          e.preventDefault();
+          tail.focus();
+        }
+      } else {
+        if (current === tail) {
+          e.preventDefault();
+          head.focus();
+        }
+      }
+    };
+    document.addEventListener("keydown", handler);
+    return () => {
+      document.removeEventListener("keydown", handler);
+      previouslyFocused?.focus();
+    };
+  }, [active, containerRef]);
+}
+
+export function useBodyScrollLock(active: boolean) {
+  useEffect(() => {
+    if (!active) return;
+    const prev = document.body.style.overflow;
+    document.body.style.overflow = "hidden";
+    return () => {
+      document.body.style.overflow = prev;
+    };
+  }, [active]);
+}