feat(i18n): #352 다국어 뼈대 ko/en/ja/es

- next-intl 5.x 도입
- src/i18n/config.ts: LOCALES 상수, detectBrowserLocale, LOCALE_LABELS(국기/네이티브명)
- src/i18n/LocaleProvider.tsx: NextIntlClientProvider wrap + localStorage tasteby_locale 저장
- src/messages/{ko,en,ja,es}.json: 초기 30개 키 (header/actions/filter/restaurant/review 5 카테고리)
- src/components/LanguageSwitcher.tsx: 헤더용 드롭다운 (국기 + native, ARIA listbox, 44px 터치)
- providers.tsx: LocaleProvider로 AuthProvider 감싸기
- page.tsx 헤더에 LanguageSwitcher 배치

설계서: docs/design/352-i18n-skeleton/README.md (Approved)

언어 선택 근거:
- ko: 기본
- en: 글로벌 1순위
- ja: 일본 사용자 + 한국 음식 관광
- es: 5억 화자, 라틴아메리카 + 스페인 확장

미번역 키는 ko fallback. URL 라우팅(/en/)/SEO meta/사용자 콘텐츠 번역은 후속.

Refs: #352

frontend/src/components/LanguageSwitcher.tsx

@@ -0,0 +1,57 @@
+"use client";
+
+import { useState } from "react";
+import { useLocale } from "@/i18n/LocaleProvider";
+import { LOCALES, LOCALE_LABELS } from "@/i18n/config";
+
+// #352 — 헤더용 언어 전환 드롭다운. 44px 터치 영역 + ARIA listbox 패턴.
+export default function LanguageSwitcher() {
+  const { locale, setLocale } = useLocale();
+  const [open, setOpen] = useState(false);
+
+  return (
+    <div className="relative">
+      <button
+        onClick={() => setOpen((v) => !v)}
+        aria-haspopup="listbox"
+        aria-expanded={open}
+        aria-label="언어 선택 / Select language"
+        className="min-h-[44px] min-w-[44px] px-2 py-1.5 flex items-center gap-1 text-sm rounded-lg hover:bg-brand-50 touch-manipulation"
+      >
+        <span aria-hidden="true">{LOCALE_LABELS[locale].flag}</span>
+        <span className="text-xs text-gray-500 uppercase">{locale}</span>
+      </button>
+      {open && (
+        <>
+          <button
+            aria-hidden="true"
+            tabIndex={-1}
+            className="fixed inset-0 z-40 cursor-default"
+            onClick={() => setOpen(false)}
+          />
+          <ul
+            role="listbox"
+            aria-label="언어 목록"
+            className="absolute right-0 mt-1 bg-surface rounded-lg shadow-lg border border-gray-200 dark:border-gray-700 py-1 z-50 min-w-[160px]"
+          >
+            {LOCALES.map((l) => (
+              <li key={l}>
+                <button
+                  role="option"
+                  aria-selected={l === locale}
+                  onClick={() => { setLocale(l); setOpen(false); }}
+                  className={`w-full text-left px-3 py-2 text-sm flex items-center gap-2 hover:bg-brand-50 ${
+                    l === locale ? "font-semibold text-brand-700" : "text-gray-700"
+                  }`}
+                >
+                  <span aria-hidden="true">{LOCALE_LABELS[l].flag}</span>
+                  <span>{LOCALE_LABELS[l].native}</span>
+                </button>
+              </li>
+            ))}
+          </ul>
+        </>
+      )}
+    </div>
+  );
+}