Dashboard: per-host resource overview bar (0.20.0)

The dashboard resource bar (CPU cores, memory used/total, containers, Docker
version) is now rendered per host instead of once for the local host — each
host section (local + each agent) shows its own bar above its stacks table.

- agent_app.py: _system_info() now also returns mem_used (from meminfo
  available), alongside the cpu_cores/mem_total added in 0.19.0.
- Frontend: extracted a ResourceBar component used by the local section and each
  AgentDashboardSection; AgentSystem type gained mem_used.

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

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.19.0",
+  "version": "0.20.0",
   "type": "module",
   "scripts": {
     "dev": "vite",