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>

components/update-toast.tsx

@@ -0,0 +1,49 @@
+"use client";
+
+import { useAtom } from "jotai";
+import { useEffect } from "react";
+import { toast } from "sonner";
+import { updateToastShownAtom } from "@/lib/atoms/update-check";
+
+export function UpdateToast() {
+  const [shown, setShown] = useAtom(updateToastShownAtom);
+
+  useEffect(() => {
+    if (shown) return;
+
+    async function check() {
+      try {
+        const res = await fetch("/api/admin/update-check");
+        if (!res.ok) return;
+
+        const data = (await res.json()) as {
+          updateAvailable: boolean;
+          currentVersion: string;
+          latestVersion: string | null;
+          releaseUrl: string | null;
+        };
+
+        if (data.updateAvailable) {
+          setShown(true);
+          toast.info(`Sofa v${data.latestVersion} is available`, {
+            description: `You're running v${data.currentVersion}.`,
+            duration: 15_000,
+            action: data.releaseUrl
+              ? {
+                  label: "View release",
+                  onClick: () =>
+                    window.open(data.releaseUrl as string, "_blank"),
+                }
+              : undefined,
+          });
+        }
+      } catch {
+        // Silently ignore network errors
+      }
+    }
+
+    check();
+  }, [shown, setShown]);
+
+  return null;
+}