refactor(frontend): replace axios with the native Fetch API

Drop the axios (and qs) dependencies in favor of a native fetch wrapper.
axios only ever handled same-origin JSON/form calls, a CSRF header, a 401
redirect, and a 403-retry, all of which the platform now provides directly.

- New src/api/http-init.ts (replaces axios-init.ts) reimplements the
  request/response interceptors on fetch: base-path prefixing,
  X-Requested-With, same-origin credentials, the CSRF token on unsafe
  methods, a single 403 retry with token refresh, and the 401
  redirect-and-latch. A small encodeForm() reproduces qs's
  arrayFormat:'repeat' encoding, so the request wire format is unchanged.
- HttpUtil (src/utils/index.ts) keeps its public signatures and the Msg
  envelope, so the ~49 API call sites are untouched. HttpOptions is now
  hand-rolled instead of extending AxiosRequestConfig.
- PanelUpdateModal drops its lone direct axios.get in favor of HttpUtil.get
  with { silent, timeout }.
- Add tests for the fetch core (CSRF header, form/JSON/FormData bodies,
  base-path prefix, 403 retry, 401 redirect, tolerant body parse) and for
  HttpUtil's envelope unwrap / toast / error mapping; this logic was
  previously untested.
- Remove the vendor-axios manualChunks branch and the qs type shim, and
  reword stale "axios" mentions in docs and route comments.

frontend/src/entries/login.tsx

@@ -2,14 +2,14 @@ import { createRoot } from 'react-dom/client';
 import { message } from 'antd';
 import 'antd/dist/reset.css';
 
-import { setupAxios } from '@/api/axios-init';
+import { setupHttp } from '@/api/http-init';
 import { applyDocumentTitle } from '@/utils';
 import { readyI18n } from '@/i18n/react';
 import { ThemeProvider } from '@/hooks/useTheme';
 import { QueryProvider } from '@/api/QueryProvider';
 import LoginPage from '@/pages/login/LoginPage';
 
-setupAxios();
+setupHttp();
 applyDocumentTitle();
 
 const messageContainer = document.getElementById('message');