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>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.16.0",
+  "version": "0.17.0",
   "type": "module",
   "scripts": {
     "dev": "vite",