Add version update check feature for admin users

Checks current version against latest GitHub release every 6 hours via
cron job, caches result in appSettings, and surfaces updates through a
toast notification (once per browser session) and an animated badge in
the settings footer. Includes an admin toggle to enable/disable checks
(enabled by default). Also renames ServerSection to RegistrationSection
now that update checks are in their own card.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/atoms/update-check.ts

@@ -0,0 +1,7 @@
+import { atomWithStorage, createJSONStorage } from "jotai/utils";
+
+export const updateToastShownAtom = atomWithStorage<boolean>(
+  "sofa:update-toast-shown",
+  false,
+  createJSONStorage(() => sessionStorage),
+);