feat(login): surface SSO callback failures via error query param

The CAS / OIDC / SAML callbacks now redirect to `/login?error=<code>`
on failure instead of letting the browser land on a raw JSON error
page, so the actionable copy reaches the user. Two codes are
recognised:

* `source_conflict` — incoming SSO username collides with an existing
  account from a different source. Message points the user at an
  administrator to link or convert.
* `auth_failed` — anything else (bad ticket, expired state, IdP
  unreachable, malformed response). Generic message: try again or
  contact the administrator.

On mount the login form picks up the `?error=` query param, maps it
through a small `messageIdByCode` table to an i18n key, and routes
the result through the existing auth-error toast. Unknown codes are
silently ignored so a future server release adding a code doesn't
render a bare key. The query param is cleared via
`history.replaceState` so a refresh doesn't re-fire the toast.

Strings added to all five locales.

src/locales/zh-CN/common.ts

@@ -257,6 +257,9 @@ export default {
   'common.select.count': '已选 {count} 项',
   'common.login.auth': '认证中...',
   'common.login.auth.failed': '认证失败',
+  'common.login.error.source_conflict':
+    '已存在同名账号但来源不同。请联系管理员关联或转换该账号。',
+  'common.login.error.auth_failed': '身份提供商认证失败。请重试或联系管理员。',
   'common.login.password': '使用密码登录',
   'common.login.username.holder': '请输入用户名',
   'common.login.password.holder': '请输入密码',