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>

backend/routers/agents.py

@@ -645,6 +645,57 @@ async def agent_image_check(
     return result
 
 
+# --------------------------------------------------------------------------- #
+# Volumes (proxied)
+# --------------------------------------------------------------------------- #
+
+
+@router.get("/{agent_id}/volumes")
+async def agent_volumes(
+    agent_id: int,
+    session: Session = Depends(get_session),
+    _user: User = Depends(get_current_user),
+) -> list[dict]:
+    agent = _get_or_404(session, agent_id)
+    return await _proxy(session, agent, "GET", "/agent/volumes") or []
+
+
+@router.post("/{agent_id}/volumes/prune")
+async def agent_volumes_prune(
+    agent_id: int,
+    request: Request,
+    session: Session = Depends(get_session),
+    user: User = Depends(require_admin),
+) -> dict:
+    agent = _get_or_404(session, agent_id)
+    result = await _proxy(session, agent, "POST", "/agent/volumes/prune")
+    audit_service.record(
+        session, user=user.username, action="agent.volume.prune", target=agent.name,
+        detail=str(result.get("VolumesDeleted") or []), ip=_ip(request),
+    )
+    return result
+
+
+@router.delete("/{agent_id}/volumes/{name}")
+async def agent_volume_delete(
+    agent_id: int,
+    name: str,
+    request: Request,
+    force: bool = Query(False),
+    session: Session = Depends(get_session),
+    user: User = Depends(require_admin),
+) -> dict:
+    agent = _get_or_404(session, agent_id)
+    result = await _proxy(
+        session, agent, "DELETE", f"/agent/volumes/{name}", params={"force": force}
+    )
+    audit_service.record(
+        session, user=user.username, action="agent.volume.delete",
+        target=f"{agent.name}/{name}", ip=_ip(request),
+    )
+    return result
+
+
 # --------------------------------------------------------------------------- #
 # File browser (proxied)
 # --------------------------------------------------------------------------- #