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/App.tsx

@@ -9,6 +9,7 @@ import { StackEditor } from "@/pages/StackEditor";
 import { RemoteStackDetail } from "@/pages/RemoteStackDetail";
 import { Images } from "@/pages/Images";
 import { Files } from "@/pages/Files";
+import { Volumes } from "@/pages/Volumes";
 import { Templates } from "@/pages/Templates";
 import { Settings } from "@/pages/Settings";
 import { Audit } from "@/pages/Audit";
@@ -48,6 +49,7 @@ export default function App() {
             <Route path="/hosts/:agentId/stacks/:id" element={<RemoteStackDetail />} />
             <Route path="/networks" element={<Networks />} />
             <Route path="/images" element={<Images />} />
+            <Route path="/volumes" element={<Volumes />} />
             <Route path="/files" element={<Files />} />
             <Route path="/templates" element={<Templates />} />
             <Route path="/audit" element={<Audit />} />