Default first-run UI to Simplified Chinese

The app now starts I18nProvider in zh-CN when no saved language exists, while preserving explicit saved choices. The login-required Ant Design modal also uses the v6 mask.closable shape so the console no longer warns about maskClosable.

Constraint: First-run locale should not override an existing saved language preference
Constraint: Ant Design 6 deprecates maskClosable on Modal.confirm
Rejected: Keep browser-language detection | first entry needs deterministic Simplified Chinese
Confidence: high
Scope-risk: narrow
Directive: Keep provider-less useI18n fallback stable for isolated tests unless all callers are wrapped in I18nProvider
Tested: npm run test:run -- src/i18n/__tests__/index.test.tsx src/components/__tests__/TemplateExplorerView.test.tsx src/components/__tests__/Header.test.tsx src/utils/__tests__/loginRequiredModal.test.ts src/utils/__tests__/authFetch.test.ts src/store/__tests__/userStore.test.ts
Tested: npm run build
Not-tested: npm run lint | existing Next 16 next lint script resolves to nonexistent /lint directory

src/i18n/__tests__/index.test.tsx

@@ -0,0 +1,50 @@
+import { render, screen, waitFor } from "@testing-library/react";
+import { beforeEach, describe, expect, it } from "vitest";
+import { I18nProvider, useI18n } from "@/i18n";
+
+function LanguageProbe() {
+  const { language, t } = useI18n();
+  return (
+    <div>
+      <span data-testid="language">{language}</span>
+      <span data-testid="label">{t("language.label")}</span>
+    </div>
+  );
+}
+
+describe("I18nProvider", () => {
+  beforeEach(() => {
+    window.localStorage.clear();
+    document.documentElement.lang = "";
+  });
+
+  it("defaults first-time visitors to Simplified Chinese", async () => {
+    render(
+      <I18nProvider>
+        <LanguageProbe />
+      </I18nProvider>
+    );
+
+    expect(screen.getByTestId("language")).toHaveTextContent("zh-CN");
+    expect(screen.getByTestId("label")).toHaveTextContent("语言");
+    await waitFor(() => {
+      expect(window.localStorage.getItem("popiart-node-language")).toBe("zh-CN");
+      expect(document.documentElement.lang).toBe("zh-CN");
+    });
+  });
+
+  it("keeps an existing saved language preference", async () => {
+    window.localStorage.setItem("popiart-node-language", "ja");
+
+    render(
+      <I18nProvider>
+        <LanguageProbe />
+      </I18nProvider>
+    );
+
+    await waitFor(() => {
+      expect(screen.getByTestId("language")).toHaveTextContent("ja");
+      expect(screen.getByTestId("label")).toHaveTextContent("言語");
+    });
+  });
+});

src/i18n/index.tsx

@@ -8,6 +8,7 @@ type Dictionary = typeof en;
 export type TranslationKey = keyof Dictionary;
 
 const STORAGE_KEY = "popiart-node-language";
+const DEFAULT_LANGUAGE: Language = "zh-CN";
 
 export const LANGUAGE_OPTIONS: { value: Language; label: string }[] = [
   { value: "en", label: "English" },
@@ -2442,15 +2443,6 @@ const dictionaries: Record<Language, Dictionary> = {
   ja,
 };
 
-function detectLanguage(): Language {
-  if (typeof navigator === "undefined") return "en";
-  const language = navigator.language.toLowerCase();
-  if (language.startsWith("ja")) return "ja";
-  if (language.includes("tw") || language.includes("hk") || language.includes("hant")) return "zh-TW";
-  if (language.startsWith("zh")) return "zh-CN";
-  return "en";
-}
-
 function interpolate(text: string, values?: Record<string, string | number>) {
   if (!values) return text;
   return text.replace(/\{(\w+)\}/g, (_, key) => String(values[key] ?? `{${key}}`));
@@ -2465,11 +2457,11 @@ interface I18nContextValue {
 const I18nContext = createContext<I18nContextValue | null>(null);
 
 export function I18nProvider({ children }: { children: React.ReactNode }) {
-  const [language, setLanguageState] = useState<Language>("en");
+  const [language, setLanguageState] = useState<Language>(DEFAULT_LANGUAGE);
 
   useEffect(() => {
     const stored = window.localStorage.getItem(STORAGE_KEY) as Language | null;
-    setLanguageState(stored && stored in dictionaries ? stored : detectLanguage());
+    setLanguageState(stored && stored in dictionaries ? stored : DEFAULT_LANGUAGE);
   }, []);
 
   useEffect(() => {

src/utils/__tests__/loginRequiredModal.test.ts

@@ -0,0 +1,26 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+describe("openLoginRequiredModal", () => {
+  beforeEach(() => {
+    vi.resetModules();
+  });
+
+  it("uses the Ant Design 6 mask.closable option instead of deprecated maskClosable", async () => {
+    const confirm = vi.fn(() => ({
+      destroy: vi.fn(),
+      update: vi.fn(),
+    }));
+
+    vi.doMock("antd", () => ({
+      Modal: { confirm },
+    }));
+
+    const { openLoginRequiredModal } = await import("@/utils/loginRequiredModal");
+    openLoginRequiredModal();
+
+    expect(confirm).toHaveBeenCalledWith(expect.objectContaining({
+      mask: { closable: false },
+    }));
+    expect(confirm.mock.calls[0][0]).not.toHaveProperty("maskClosable");
+  });
+});

src/utils/loginRequiredModal.ts

@@ -29,7 +29,7 @@ export function openLoginRequiredModal() {
     width: 360,
     icon: null,
     keyboard: false,
-    maskClosable: false,
+    mask: { closable: false },
     onOk: () => {
       window.location.assign(LOGIN_REDIRECT_URL);
     },