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/agent_app.py

@@ -44,6 +44,7 @@ from services import (
     image_service,
     network_service,
     update_service,
+    volume_service,
 )
 
 logger = logging.getLogger("stackpilot.agent")
@@ -59,7 +60,7 @@ def _map_docker(exc: DockerError):
         raise HTTPException(status_code=code, detail=exc.detail or exc.error)
     raise exc  # falls through to the global 502 DockerError handler
 
-AGENT_VERSION = "0.16.0"
+AGENT_VERSION = "0.17.0"
 
 
 # --------------------------------------------------------------------------- #
@@ -436,6 +437,36 @@ async def image_check() -> dict:
     return await update_service.check_all()
 
 
+# --------------------------------------------------------------------------- #
+# Volumes
+# --------------------------------------------------------------------------- #
+
+
+@app.get("/agent/volumes", dependencies=[Depends(verify_token)])
+def list_volumes() -> list[dict]:
+    return volume_service.list_volumes()
+
+
+@app.delete("/agent/volumes/{name}", dependencies=[Depends(verify_token)])
+def delete_volume(name: str, force: bool = Query(False)) -> dict:
+    vols = {v["name"]: v for v in volume_service.list_volumes()}
+    if name in vols and vols[name]["in_use"] and not force:
+        raise HTTPException(
+            status_code=409,
+            detail={
+                "error": "volume_in_use",
+                "detail": f"Volume '{name}' is used by: {', '.join(vols[name]['used_by'])}",
+            },
+        )
+    volume_service.remove_volume(name, force=force)
+    return {"ok": True}
+
+
+@app.post("/agent/volumes/prune", dependencies=[Depends(verify_token)])
+def prune_volumes() -> dict:
+    return volume_service.prune_volumes()
+
+
 # --------------------------------------------------------------------------- #
 # File browser (sandboxed by this agent's ALLOWED_BROWSE_ROOTS/HOST_ROOT_PREFIX)
 # --------------------------------------------------------------------------- #