Volumes page: on-demand volume sizes (0.18.0)

Docker's volume list has no size, so add a "Compute sizes" button that runs
`docker system df` (via client.df()) and shows per-volume size in a new Size
column. The df walk is expensive (seconds), so results are cached ~60s and
loaded on demand instead of on every poll.

- volume_service.volume_sizes(force) with a 60s TTL cache; GET /api/volumes/sizes
  + agent /agent/volumes/sizes + proxy /api/agents/{id}/volumes/sizes.
- Frontend: volumesApi.sizes(force, agentId); Volumes page gained a Size column
  and a Compute sizes button (per host) that triggers the lookup.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/routers/agents.py

@@ -660,6 +660,19 @@ async def agent_volumes(
     return await _proxy(session, agent, "GET", "/agent/volumes") or []
 
 
+@router.get("/{agent_id}/volumes/sizes")
+async def agent_volume_sizes(
+    agent_id: int,
+    force: bool = Query(False),
+    session: Session = Depends(get_session),
+    _user: User = Depends(get_current_user),
+) -> dict:
+    agent = _get_or_404(session, agent_id)
+    return await _proxy(
+        session, agent, "GET", "/agent/volumes/sizes", params={"force": force}
+    )
+
+
 @router.post("/{agent_id}/volumes/prune")
 async def agent_volumes_prune(
     agent_id: int,