feat(xai): add Grok account management UI with four-locale strings

Add XaiOAuthSection (device-code login, account list, default selection)
backed by the generalized useManagedAuth hook, wire it into the Auth
Center and both Claude provider forms, and expose model fetching for
signed-in accounts.

- Accounts requiring re-auth stay visible in the account selector as
  disabled items with an expired badge instead of silently vanishing.
- Auth status refetches periodically so a revoked refresh token surfaces
  without reloading the panel.
- All strings ship in zh/en/ja/zh-TW; a locale coverage test pins every
  required key in all four locales.

tests/components/XaiOAuthSection.test.tsx

@@ -0,0 +1,63 @@
+import { render, screen } from "@testing-library/react";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { XaiOAuthSection } from "@/components/providers/forms/XaiOAuthSection";
+
+const mockUseXaiOauth = vi.hoisted(() => vi.fn());
+
+vi.mock("@/components/providers/forms/hooks/useXaiOauth", () => ({
+  useXaiOauth: mockUseXaiOauth,
+}));
+
+describe("XaiOAuthSection", () => {
+  beforeEach(() => {
+    mockUseXaiOauth.mockReturnValue({
+      accounts: [
+        {
+          id: "expired-account",
+          login: "expired@example.com",
+          avatar_url: null,
+          authenticated_at: 1,
+          github_domain: "x.ai",
+          requires_reauth: true,
+        },
+        {
+          id: "usable-account",
+          login: "usable@example.com",
+          avatar_url: null,
+          authenticated_at: 2,
+          github_domain: "x.ai",
+          requires_reauth: false,
+        },
+      ],
+      defaultAccountId: "usable-account",
+      hasAnyAccount: true,
+      isAuthenticated: true,
+      pollingState: "idle",
+      deviceCode: null,
+      error: null,
+      isPolling: false,
+      isAddingAccount: false,
+      isRemovingAccount: false,
+      isSettingDefaultAccount: false,
+      addAccount: vi.fn(),
+      removeAccount: vi.fn(),
+      setDefaultAccount: vi.fn(),
+      cancelAuth: vi.fn(),
+      logout: vi.fn(),
+    });
+  });
+
+  it("keeps a selected account visible when it requires reauthentication", () => {
+    render(
+      <XaiOAuthSection
+        selectedAccountId="expired-account"
+        onAccountSelect={vi.fn()}
+      />,
+    );
+
+    expect(screen.getByRole("combobox")).toHaveTextContent(
+      "expired@example.com",
+    );
+    expect(screen.getByRole("combobox")).toHaveTextContent("凭据已失效");
+  });
+});