Dashboard: per-host disk usage (0.21.0)

The per-host resource bar gained a Disk stat (used / total). The agent now
reports disk_total/disk_used via shutil.disk_usage on its stacks dir (a host
bind-mount), alongside the existing cpu/mem/containers fields; the local host
uses the existing system info disk data.

- agent_app.py: _disk_info() + disk_total/disk_used in _system_info().
- Frontend: ResourceBar gained diskUsed/diskTotal (5-column grid); AgentSystem
  type gained disk_total/disk_used.

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

README.md

@@ -152,11 +152,13 @@ as intuitive as Dockge, as capable as Portainer for Compose workflows.
 
 - 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.
+  used/total, disk used/total, containers, Docker version) and a
+  **stacks-with-usage table** with 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`,
-  `mem_total` and `mem_used` so the remote resource bar and meters have a host
+  `mem_total`, `mem_used`, and `disk_total`/`disk_used` (disk of the host volume
+  backing the stacks dir) so the remote resource bar and meters have a host
   reference.
 
 ### Phase 16 — Volumes page (multi-host)