feat(web): add backend switcher and connection status

Split the login page into backend list and credentials, show connection status in the sidebar and top bar, and render dialogs in a portal.

web/shared/api/client.ts

@@ -57,6 +57,28 @@ async function request<T>(method: string, path: string, body?: unknown, init?: R
   return data as T
 }
 
+/**
+ * 在另一个后端上用用户名和密码登录：不设 Cookie（跨域用不了），把会话令牌放在响应里返回（`return_session`），
+ * 调用方存进浏览器，之后每个请求带上。
+ */
+export async function loginTo(base: string, username: string, password: string): Promise<string> {
+  const response = await fetch(`${base}/api/admin/auth/login`, {
+    method: 'POST',
+    headers: { Accept: 'application/json', 'Content-Type': 'application/json' },
+    credentials: 'omit',
+    body: JSON.stringify({ username, password, return_session: true }),
+  })
+  const text = await response.text()
+  const data = text ? (JSON.parse(text) as unknown) : undefined
+  if (!response.ok) {
+    const problem = (data ?? {}) as { code?: string; detail?: string; title?: string }
+    throw new ApiError(response.status, problem.code ?? 'error', problem.detail ?? problem.title ?? response.statusText)
+  }
+  const token = (data as { session_token?: string | null } | undefined)?.session_token
+  if (!token) throw new ApiError(502, 'no_session', '这个后端的版本还不支持跨域登录，请先升级后端')
+  return token
+}
+
 export const api = {
   get: <T>(path: string, query?: Query, init?: RequestInit) => request<T>('GET', withQuery(path, query), undefined, init),
   post: <T>(path: string, body?: unknown) => request<T>('POST', path, body ?? {}),