0.36.0: per-stack image-update indicator on the Stacks overview

Shows an amber "Update" pill next to a stack's status (and highlights the
inline Update button) when any of the stack's images has a newer digest in
the registry. Reuses the existing background image-update check — a new
update_service.stacks_update_summary() reads the cached digests in a single
container sweep (no extra registry calls), exposed as GET /api/stacks/updates
and proxied per agent at GET /api/agents/{id}/stacks/updates. The Stacks page
and each remote-host section poll it every 60s.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/pages/Stacks.tsx

@@ -43,6 +43,11 @@ export function Stacks() {
     queryFn: stacksApi.stats,
     refetchInterval: 5000,
   });
+  const updates = useQuery({
+    queryKey: ["stack-updates"],
+    queryFn: stacksApi.updates,
+    refetchInterval: 60000,
+  });
   const info = useQuery({
     queryKey: ["system"],
     queryFn: systemApi.info,
@@ -127,6 +132,7 @@ export function Stacks() {
         <StacksTable
           stacks={filtered}
           stats={stats.data}
+          updates={updates.data}
           hostCpus={info.data?.cpu_cores ?? 0}
           hostMem={info.data?.ram.total ?? 0}
           isAdmin={isAdmin}