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/api/volumes.ts

@@ -1,13 +1,17 @@
 import api from "./client";
 import type { HostPathResult, VolumeInfo } from "@/types";
 
+// Base path for the local host or, when agentId is given, a remote agent.
+const base = (agentId?: number) =>
+  agentId != null ? `/api/agents/${agentId}/volumes` : "/api/volumes";
+
 export const volumesApi = {
-  list: () => api.get<VolumeInfo[]>("/api/volumes").then((r) => r.data),
-  orphaned: () =>
-    api.get<VolumeInfo[]>("/api/volumes/orphaned").then((r) => r.data),
-  remove: (name: string, force = false) =>
-    api.delete(`/api/volumes/${name}?force=${force}`).then((r) => r.data),
-  prune: () => api.post("/api/volumes/prune").then((r) => r.data),
+  list: (agentId?: number) =>
+    api.get<VolumeInfo[]>(base(agentId)).then((r) => r.data),
+  remove: (name: string, force = false, agentId?: number) =>
+    api.delete(`${base(agentId)}/${name}?force=${force}`).then((r) => r.data),
+  prune: (agentId?: number) =>
+    api.post<{ VolumesDeleted: string[] | null }>(`${base(agentId)}/prune`).then((r) => r.data),
   generateYaml: (spec: Record<string, unknown>) =>
     api
       .post<{ yaml: string }>("/api/volumes/generate-yaml", spec)