Dashboard: surface the real compute_fleet error in the response (0.38.2)

The fleet endpoint returned a bare 500, so the error banner only showed
"status code 500" with no cause. Wrap the call to log the full traceback
server-side and return the exception type, message and originating
file:line in the HTTP detail, so the dashboard banner pinpoints the
failure for an authenticated user.

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

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.38.1",
+  "version": "0.38.2",
   "type": "module",
   "scripts": {
     "dev": "vite",