Dashboard: surface fleet load errors instead of infinite skeletons (0.38.1)

The cockpit cards gated purely on `fleet.data`, so any failed
/api/dashboard/fleet request (e.g. a stale backend returning 404, or a
500) left the new components stuck on skeletons forever. Render a clear
error banner with the API message and a Retry button when the query
errors with no data, so the actual cause is visible.

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

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.38.0",
+  "version": "0.38.1",
   "type": "module",
   "scripts": {
     "dev": "vite",