refactor(web): 拆 lib/api.ts 789→109 barrel + lib/api/ 8域文件(纯搬移零逻辑)

- lib/api/{core,auth,chat,sessions,personas,admin,adminChannels,adminSystem}.ts
- api.ts 改纯 barrel 显式 re-export(export type for types, isolatedModules); 导出名 77→77, 36处导入点零改动
- 代码行多重集比对一致(仅 request/limitQuery 各加一 export 供兄弟模块); 每文件≤500/函数≤80
- 前端 build 绿(165 modules)

frontend/web/src/lib/api/auth.ts

@@ -0,0 +1,19 @@
+import type { LoginResult } from "../types";
+import { request } from "./core";
+
+/** POST /api/v1/auth/login — exchanges user/password for a JWT. */
+export function login(username: string, password: string): Promise<LoginResult> {
+  return request<LoginResult>(
+    "/auth/login",
+    {
+      method: "POST",
+      body: JSON.stringify({ username, password }),
+    },
+    { skipAuthExpired: true }
+  );
+}
+
+/** GET /api/v1/health — liveness probe. */
+export function health(): Promise<{ status: string }> {
+  return request<{ status: string }>("/health");
+}