feat(ui): P4-4 별점 공통화 + 로그인 모달 접근성 (#281+#283)

#281 (리뷰/메모 UI):
- Stars 컴포넌트 신규 (lib 분리 가능한 공통 별점) — 0.5 단위 절반 채우기 시각 구분
- ReviewSection/MemoSection의 StarDisplay 제거 → 공통 Stars 사용 (시각 일관성)
- StarSelector: role='radiogroup'/role='radio' + aria-checked, 44×44px 터치 영역,
  반쪽 별 '⯨' 표시로 시각 차별화
- ReviewSection/MemoSection: API 실패 try/catch + alert 사용자 피드백
- MyReviewsList: Math.round 별점 → Stars 0.5단위 정확 렌더

#283 (로그인 메뉴):
- LoginMenu: useEscapeKey + useFocusTrap + useBodyScrollLock 적용
- role='dialog' / aria-modal / aria-labelledby / aria-label='로그인 창 닫기'
- onError 콘솔만 → 인라인 role='alert' 메시지로 사용자 피드백
- max-w-xs → max-w-sm (위젯 260px + 패딩 24px = 308px 안전 수용)

후속 분리:
- #343 (next/image + ARIA Tabs + Stars 테스트)
- #344 (z-index 토큰 + i18n)

Refs: #281 #283

frontend/src/components/LoginMenu.tsx

@@ -1,8 +1,9 @@
 "use client";
 
-import { useState } from "react";
+import { useRef, useState } from "react";
 import { createPortal } from "react-dom";
 import { GoogleLogin } from "@react-oauth/google";
+import { useEscapeKey, useFocusTrap, useBodyScrollLock } from "@/lib/hooks/useModalA11y";
 
 interface LoginMenuProps {
   onGoogleSuccess: (credential: string) => void;
@@ -10,6 +11,22 @@ interface LoginMenuProps {
 
 export default function LoginMenu({ onGoogleSuccess }: LoginMenuProps) {
   const [open, setOpen] = useState(false);
+  const [errorMsg, setErrorMsg] = useState<string | null>(null);
+  const dialogRef = useRef<HTMLDivElement>(null);
+  const titleId = "login-dialog-title";
+
+  // #283 — 모달 접근성: ESC / focus trap / body scroll lock
+  useEscapeKey(open, () => setOpen(false));
+  useFocusTrap(open, dialogRef);
+  useBodyScrollLock(open);
+
+  const handleSuccess = (res: { credential?: string }) => {
+    setErrorMsg(null);
+    if (res.credential) {
+      onGoogleSuccess(res.credential);
+      setOpen(false);
+    }
+  };
 
   return (
     <>
@@ -26,26 +43,34 @@ export default function LoginMenu({ onGoogleSuccess }: LoginMenuProps) {
           style={{ zIndex: 99999 }}
           onClick={(e) => { if (e.target === e.currentTarget) setOpen(false); }}
         >
-          <div className="bg-surface rounded-2xl shadow-2xl p-6 mx-4 w-full max-w-xs space-y-4">
+          <div
+            ref={dialogRef}
+            role="dialog"
+            aria-modal="true"
+            aria-labelledby={titleId}
+            tabIndex={-1}
+            className="bg-surface rounded-2xl shadow-2xl p-6 mx-4 w-full max-w-sm space-y-4"
+          >
             <div className="flex items-center justify-between">
-              <h3 className="text-base font-semibold dark:text-gray-100">로그인</h3>
+              <h3 id={titleId} className="text-base font-semibold dark:text-gray-100">로그인</h3>
               <button
                 onClick={() => setOpen(false)}
-                className="text-gray-400 hover:text-gray-600 dark:hover:text-gray-200 text-lg leading-none"
+                aria-label="로그인 창 닫기"
+                className="text-gray-400 hover:text-gray-600 dark:hover:text-gray-200 text-lg leading-none p-2 -m-2"
               >
                 ✕
               </button>
             </div>
             <p className="text-xs text-gray-400 dark:text-gray-500">소셜 계정으로 간편 로그인</p>
+            {errorMsg && (
+              <p role="alert" className="text-xs text-red-600 dark:text-red-400 bg-red-50 dark:bg-red-950/40 rounded p-2">
+                {errorMsg}
+              </p>
+            )}
             <div className="flex flex-col items-center gap-3">
               <GoogleLogin
-                onSuccess={(res) => {
-                  if (res.credential) {
-                    onGoogleSuccess(res.credential);
-                    setOpen(false);
-                  }
-                }}
-                onError={() => console.error("Google login failed")}
+                onSuccess={handleSuccess}
+                onError={() => setErrorMsg("Google 로그인에 실패했습니다. 팝업 차단 또는 네트워크 상태를 확인해주세요.")}
                 size="large"
                 width="260"
                 text="signin_with"