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/src/components/layout/Sidebar.tsx

@@ -4,6 +4,7 @@ import {
   Boxes,
   Network,
   Image,
+  Database,
   FolderTree,
   LayoutTemplate,
   ScrollText,
@@ -23,6 +24,7 @@ const nav = [
   { to: "/stacks", label: "Stacks", icon: Boxes },
   { to: "/networks", label: "Networks", icon: Network },
   { to: "/images", label: "Images", icon: Image },
+  { to: "/volumes", label: "Volumes", icon: Database },
   { to: "/files", label: "Files", icon: FolderTree },
   { to: "/templates", label: "Templates", icon: LayoutTemplate },
   { to: "/audit", label: "Audit log", icon: ScrollText },