feat: Docker diagnostic hints on disconnection

- Classify Docker errors into categories (socket_not_found, connection_refused,
  permission_denied, timeout, tls_error) with platform-specific hints
- Enrich GET /api/health with structured diagnostics (category, hints, platform)
- Expandable hints panel in sidebar when Docker is disconnected
- "Retry now" button for immediate re-check
- Collapsible raw error details for advanced users

web/src/lib/i18n/en.json

@@ -5,7 +5,9 @@
   },
   "health": {
     "connected": "connected",
-    "disconnected": "disconnected"
+    "disconnected": "disconnected",
+    "rawError": "Technical details",
+    "retryNow": "Retry now"
   },
   "nav": {
     "dashboard": "Dashboard",