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>

app/api/admin/update-check/route.ts

@@ -0,0 +1,15 @@
+import { headers } from "next/headers";
+import { NextResponse } from "next/server";
+import { auth } from "@/lib/auth/server";
+import { getCachedUpdateCheck } from "@/lib/services/update-check";
+
+export async function GET() {
+  const session = await auth.api.getSession({ headers: await headers() });
+  if (!session)
+    return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+  if (session.user.role !== "admin")
+    return NextResponse.json({ error: "Forbidden" }, { status: 403 });
+
+  const result = getCachedUpdateCheck();
+  return NextResponse.json(result);
+}