Phase 16: volumes page, multi-host (0.17.0)

Adds a dedicated Volumes page (sidebar) with per-host sections (local + each
online agent), matching the Networks/Images layout. Lists volumes with driver,
owning stack, in-use containers and mountpoint; admins can delete (with an
in-use warning + force option) and prune unused, plus an "only unused" filter.

- agent_app.py: /agent/volumes (list/delete with in-use 409 guard/prune)
  reusing volume_service.
- routers/agents.py: proxy routes /api/agents/{id}/volumes/* (audit-logged
  delete/prune).
- Frontend: volumesApi list/remove/prune take an optional agentId; new
  pages/Volumes.tsx (VolumesSection per host) + sidebar entry + /volumes route.
  The volume wizard (generate-yaml/host paths) stays local and unchanged.

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

README.md

@@ -9,7 +9,7 @@ as intuitive as Dockge, as capable as Portainer for Compose workflows.
 > + Phase 9 (Networks) + Phase 10 (iGPU passthrough) + Phase 11 (Remote UX &
 > network attach) + Phase 12 (File browser) + Phase 13 (Multi-host networks &
 > images) + Phase 14 (Multi-host file browser) + Phase 15 (Dashboard stack
-> resource usage) complete.
+> resource usage) + Phase 16 (Volumes page, multi-host) complete.
 
 ## What works today (Phase 1)
 
@@ -147,6 +147,17 @@ as intuitive as Dockge, as capable as Portainer for Compose workflows.
   container or connect any container on the host (`POST /api/networks/{id}/connect`
   / `/disconnect`).
 
+### Phase 16 — Volumes page (multi-host)
+
+- **New Volumes page** (sidebar) with per-host sections (local + each online
+  agent, like Networks/Images). Lists Docker volumes with driver, owning stack,
+  in-use containers and mountpoint.
+- Admin actions: delete a volume (with an in-use warning + force option) and
+  **Prune unused**; an *Only unused* filter. New agent endpoints
+  `/agent/volumes` (list/delete/prune), proxied at `/api/agents/{id}/volumes/*`.
+- The Volume **Wizard** in the stack editor (bind/named/NFS/SMB/tmpfs YAML
+  generation) is unchanged — the new page is for managing/cleaning up volumes.
+
 ### Phase 15 — Dashboard stack resource usage
 
 - **The dashboard now lists stacks in a table** (status, services) with live