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/styles/page-shell.css

@@ -5,7 +5,8 @@
 .settings-page,
 .nodes-page,
 .groups-page,
-.api-docs-page {
+.api-docs-page,
+.sponsors-page {
   --bg-page: #e6e8ec;
   --bg-card: #ffffff;
   min-height: 100vh;
@@ -19,7 +20,8 @@
 .settings-page.is-dark,
 .nodes-page.is-dark,
 .groups-page.is-dark,
-.api-docs-page.is-dark {
+.api-docs-page.is-dark,
+.sponsors-page.is-dark {
   --bg-page: #1a1b1f;
   --bg-card: #23252b;
 }
@@ -31,7 +33,8 @@
 .settings-page.is-dark.is-ultra,
 .nodes-page.is-dark.is-ultra,
 .groups-page.is-dark.is-ultra,
-.api-docs-page.is-dark.is-ultra {
+.api-docs-page.is-dark.is-ultra,
+.sponsors-page.is-dark.is-ultra {
   --bg-page: #000;
   --bg-card: #101013;
 }