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.
Jason