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.

src/components/settings/AuthCenterPanel.tsx

@@ -4,6 +4,8 @@ import { Badge } from "@/components/ui/badge";
 import { CodexIcon } from "@/components/BrandIcons";
 import { CopilotAuthSection } from "@/components/providers/forms/CopilotAuthSection";
 import { CodexOAuthSection } from "@/components/providers/forms/CodexOAuthSection";
+import { XaiOAuthSection } from "@/components/providers/forms/XaiOAuthSection";
+import { ProviderIcon } from "@/components/ProviderIcon";
 
 export function AuthCenterPanel() {
   const { t } = useTranslation();
@@ -69,6 +71,24 @@ export function AuthCenterPanel() {
 
         <CodexOAuthSection />
       </section>
+
+      <section className="rounded-xl border border-border/60 bg-card/60 p-6">
+        <div className="mb-4 flex items-center gap-3">
+          <div className="flex h-10 w-10 items-center justify-center rounded-xl bg-muted">
+            <ProviderIcon icon="xai" name="xAI" size={20} />
+          </div>
+          <div>
+            <h4 className="font-medium">xAI (Grok OAuth)</h4>
+            <p className="text-sm text-muted-foreground">
+              {t("settings.authCenter.xaiOauthDescription", {
+                defaultValue: "管理 xAI / Grok 账号",
+              })}
+            </p>
+          </div>
+        </div>
+
+        <XaiOAuthSection />
+      </section>
     </div>
   );
 }