Stop long prunes failing with a bare 500 (0.60.1)

A prune that deletes a lot of data (image prune -a in particular) easily
runs past docker-py's 60s read timeout. requests' ReadTimeout is not a
DockerException, so it slipped past safe_call and the UI only showed
"Request failed with status code 500".

Prunes now use a client with a 540s timeout (below nginx's 600s
proxy_read_timeout), and safe_call turns transport timeouts and
connection errors into DockerError, so they surface as a 502 with a
message.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.60.0",
+  "version": "0.60.1",
   "type": "module",
   "scripts": {
     "dev": "vite",