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/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.35.0",
+  "version": "0.36.0",
   "type": "module",
   "scripts": {
     "dev": "vite",