feat(panel): add sponsor slots fed from sponsors.sanaei.dev

Monthly sponsor placements need to change without cutting a panel
release. Panels now read 3X/sponsors.json from the MHSanaei/sponsors
repo (GitHub Pages on sponsors.sanaei.dev) and show active sponsors in
four slots: an overview banner, a rotating sidebar card (max three), the
login page and a new Sponsors page that also lists open placements.

An entry shows only while enable is not false and until is in the
future; links must be https and logos are png/webp/jpg by name only.
The list is cached for an hour and the last good copy survives upstream
failures; logos are proxied through /sponsors/logo/:name with failures
cached, so CSP stays 'self' and admin browsers never reach a third
party. Admins can hide a slot for 24h. Under XUI_DEBUG the panel reads
a sibling ../sponsors/3X checkout so edits can be previewed before push.

frontend/src/pages/api-docs/endpoints.ts

@@ -236,6 +236,13 @@ export const sections: readonly Section[] = [
           'Mint a CSRF token for the current session. The SPA replays it in the X-CSRF-Token header on unsafe requests. Bearer-token callers can skip this — the middleware short-circuits CSRF for authenticated API requests.',
         response: '{\n  "success": true,\n  "obj": "csrf-token-string"\n}',
       },
+      {
+        method: 'GET',
+        path: '/sponsors',
+        summary:
+          'Public. Active paid sponsor placements read from the project sponsors.json (cached for 1h); expired entries are dropped. Logos are proxied by the panel at /sponsors/logo/{name}. Used by the login page and panel sponsor slots.',
+        responseSchema: 'SponsorList',
+      },
       {
         method: 'POST',
         path: '/getTwoFactorEnable',