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/vite.config.js

@@ -286,7 +286,7 @@ export default defineConfig({
     port: 5173,
     strictPort: true,
     proxy: {
-      '^/(?:[^/]+/)?(login|logout|getTwoFactorEnable|csrf-token|panel|server)(?:/|$)': makeBackendProxy(BACKEND_TARGET),
+      '^/(?:[^/]+/)?(login|logout|getTwoFactorEnable|csrf-token|sponsors|panel|server)(?:/|$)': makeBackendProxy(BACKEND_TARGET),
       '^/$': makeBackendProxy(BACKEND_TARGET),
       '^/[^/]+/$': makeBackendProxy(BACKEND_TARGET),
       '^/(?:[^/]+/)?ws$': {