Phase 17: multi-host dashboard (0.19.0)

The dashboard now renders a stacks-with-usage table per host: the local host
plus a section for each registered agent (online dot + offline notice), reusing
the same CPU/memory meters and inline start/stop/restart actions.

- agent_app.py: GET /agent/stacks/stats (reuses stats_service); /agent/system
  now also returns cpu_cores + mem_total for remote meter references.
- routers/agents.py: proxy GET /api/agents/{id}/stacks/stats (declared before
  /{agent_id}/stacks/{stack_id}).
- Frontend: agentsApi.system + stackStats; Dashboard refactored into a shared
  StacksTable used by the local section and a per-agent AgentDashboardSection.

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

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.18.0",
+  "version": "0.19.0",
   "type": "module",
   "scripts": {
     "dev": "vite",