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/lib/api/auth.ts

@@ -1,6 +1,9 @@
 import { invoke } from "@tauri-apps/api/core";
 
-export type ManagedAuthProvider = "github_copilot" | "codex_oauth";
+export type ManagedAuthProvider =
+  | "github_copilot"
+  | "codex_oauth"
+  | "xai_oauth";
 
 export interface ManagedAuthAccount {
   id: string;
@@ -10,6 +13,7 @@ export interface ManagedAuthAccount {
   authenticated_at: number;
   is_default: boolean;
   github_domain: string;
+  requires_reauth: boolean;
 }
 
 export interface ManagedAuthStatus {

src/lib/api/model-fetch.ts

@@ -42,6 +42,15 @@ export async function fetchCodexOauthModels(
   });
 }
 
+/** 获取当前 xAI OAuth 账号可访问的模型列表。 */
+export async function fetchXaiOauthModels(
+  accountId?: string | null,
+): Promise<FetchedModel[]> {
+  return invoke("get_xai_oauth_models", {
+    accountId: accountId || null,
+  });
+}
+
 /**
  * 根据错误类型显示对应的 toast 提示
  */