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/pages/api-docs/endpoints.ts

@@ -1392,7 +1392,7 @@ export const sections: readonly Section[] = [
       {
         method: 'POST',
         path: '/panel/api/xray/outbound-subs/:id/del',
-        summary: 'Delete an outbound subscription by id (POST alias of DELETE for axios-friendly clients).',
+        summary: 'Delete an outbound subscription by id (POST alias of DELETE for clients that cannot send DELETE).',
         params: [
           { name: 'id', in: 'path', type: 'integer', desc: 'Subscription id.' },
         ],