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/volumes.py

@@ -31,6 +31,15 @@ def orphaned(_user: User = Depends(get_current_user)) -> list[dict]:
     return volume_service.orphaned_volumes()
 
 
+@router.get("/api/volumes/sizes")
+def volume_sizes(
+    force: bool = Query(False),
+    _user: User = Depends(get_current_user),
+) -> dict:
+    """Volume sizes in bytes ({name: size|null}). Expensive; cached ~60s."""
+    return volume_service.volume_sizes(force=force)
+
+
 @router.delete("/api/volumes/{name}")
 def delete_volume(
     name: str,