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>
