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>

frontend/src/api/volumes.ts

@@ -8,6 +8,10 @@ const base = (agentId?: number) =>
 export const volumesApi = {
   list: (agentId?: number) =>
     api.get<VolumeInfo[]>(base(agentId)).then((r) => r.data),
+  sizes: (force = false, agentId?: number) =>
+    api
+      .get<Record<string, number | null>>(`${base(agentId)}/sizes`, { params: { force } })
+      .then((r) => r.data),
   remove: (name: string, force = false, agentId?: number) =>
     api.delete(`${base(agentId)}/${name}?force=${force}`).then((r) => r.data),
   prune: (agentId?: number) =>