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>

README.md

@@ -150,12 +150,14 @@ as intuitive as Dockge, as capable as Portainer for Compose workflows.
 
 ### Phase 17 — Multi-host dashboard
 
-- The dashboard now shows a **stacks-with-usage table per host** — the local host
-  plus a section for each registered agent (online dot, offline notice), with the
-  same CPU/memory meters and inline start/stop/restart as the local list.
+- The dashboard now shows, **per host** (local + each registered agent, online
+  dot / offline notice), a **resource overview bar** (CPU cores, memory
+  used/total, containers, Docker version) and a **stacks-with-usage table** with
+  the same CPU/memory meters and inline start/stop/restart.
 - New agent endpoint `/agent/stacks/stats` (proxied at
-  `/api/agents/{id}/stacks/stats`); `/agent/system` now also reports `cpu_cores`
-  and `mem_total` so remote meters have a host reference.
+  `/api/agents/{id}/stacks/stats`); `/agent/system` now also reports `cpu_cores`,
+  `mem_total` and `mem_used` so the remote resource bar and meters have a host
+  reference.
 
 ### Phase 16 — Volumes page (multi-host)