fix(deploy): toast when restoring checkpoint with no snapshot

handleRestore previously returned silently when deployedConfigSnapshot
was null, leaving the user wondering why their click did nothing. Show
a warning toast explaining that the checkpoint predates snapshotting.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/src/pages/AppsTab/AppDeploymentPage/index.tsx

@@ -321,7 +321,14 @@ export default function AppDeploymentPage() {
     const deployment = deployments.find((d) => d.id === deploymentId);
     if (!deployment) return;
     const snap = deployment.deployedConfigSnapshot;
-    if (!snap) return;
+    if (!snap) {
+      toast({
+        title: 'Cannot restore checkpoint',
+        description: 'This checkpoint predates snapshotting and cannot be restored.',
+        variant: 'warning',
+      });
+      return;
+    }
 
     setForm((prev) => {
       const a = snap.agentConfig ?? {};