test(frontend): #343 Jest+RTL 인프라 + ARIA Tabs + remotePatterns

테스트 인프라:
- Jest 30 + jest-environment-jsdom + RTL + jest-dom matchers
- next/jest로 SWC/Next.js 자동 통합
- jest.config.ts (setupFilesAfterEnv) + jest.setup.ts
- npm scripts: test, test:watch
- 샘플 테스트 3개, 13/13 통과:
  - i18n/config: isLocale + detectBrowserLocale (5 케이스)
  - Stars 컴포넌트: 별점/aria/clamp/showNumber (5 케이스)
  - admin-utils: getAdminToken + authHeaders (4 케이스)

ARIA Tabs (MyReviewsList):
- role=tablist + tab + aria-selected + aria-controls + tabIndex
- panel에 role=tabpanel + aria-labelledby

next/image:
- next.config.ts remotePatterns: lh3.googleusercontent.com / i.ytimg.com / yt3.ggpht.com
- ReviewSection의 user_avatar_url에 명시적 eslint-disable + 사유

후속(별도): 전체 컴포넌트 테스트 점진 추가, 백엔드 JUnit 인프라, E2E (Playwright), CI 통합

설계서: docs/design/343-frontend-test-infra/README.md

Refs: #343

frontend/__tests__/i18n-config.test.ts

@@ -0,0 +1,42 @@
+/**
+ * #343 — i18n/config 순수 함수 단위 테스트.
+ */
+import { isLocale, detectBrowserLocale, DEFAULT_LOCALE } from "@/i18n/config";
+
+describe("i18n/config.isLocale", () => {
+  it("returns true for supported locales", () => {
+    expect(isLocale("ko")).toBe(true);
+    expect(isLocale("en")).toBe(true);
+    expect(isLocale("ja")).toBe(true);
+    expect(isLocale("es")).toBe(true);
+  });
+
+  it("returns false for unsupported / null / undefined", () => {
+    expect(isLocale("fr")).toBe(false);
+    expect(isLocale("zh")).toBe(false);
+    expect(isLocale(null)).toBe(false);
+    expect(isLocale(undefined)).toBe(false);
+    expect(isLocale("")).toBe(false);
+  });
+});
+
+describe("i18n/config.detectBrowserLocale", () => {
+  // jsdom의 navigator.language는 기본 'en-US'
+  it("returns supported locale from navigator.language", () => {
+    Object.defineProperty(navigator, "language", { value: "en-US", configurable: true });
+    expect(detectBrowserLocale()).toBe("en");
+    Object.defineProperty(navigator, "language", { value: "ko-KR", configurable: true });
+    expect(detectBrowserLocale()).toBe("ko");
+    Object.defineProperty(navigator, "language", { value: "ja", configurable: true });
+    expect(detectBrowserLocale()).toBe("ja");
+    Object.defineProperty(navigator, "language", { value: "es-MX", configurable: true });
+    expect(detectBrowserLocale()).toBe("es");
+  });
+
+  it("falls back to DEFAULT_LOCALE for unsupported", () => {
+    Object.defineProperty(navigator, "language", { value: "fr-FR", configurable: true });
+    expect(detectBrowserLocale()).toBe(DEFAULT_LOCALE);
+    Object.defineProperty(navigator, "language", { value: "zh-CN", configurable: true });
+    expect(detectBrowserLocale()).toBe(DEFAULT_LOCALE);
+  });
+});