Dashboard: per-host Docker volumes total (0.21.1)

The per-host resource bar gained a "Volumes" stat showing the total size of
that host's Docker volumes. It reuses the existing cached /volumes/sizes lookup
(docker system df, ~60s TTL) summed client-side, polled every 60s per host so
the slow df walk never blocks the fast system-info poll.

Frontend-only. Fixed sumSizes to reduce<number> so the value is number|undefined.

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

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.21.0",
+  "version": "0.21.1",
   "type": "module",
   "scripts": {
     "dev": "vite",