feat: add QR code to enrollment screen for Traccar Client setup

Generate a QR code URL containing device ID, server URL, and update
interval that the Traccar Client app can scan to auto-configure.
The enrollment modal now shows the QR prominently with manual setup
collapsed as a fallback. Also pins Traccar to 6.11 and fixes Docker
health checks (IPv6/curl issues).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docker-compose.yml

@@ -52,7 +52,7 @@ services:
 
   # Traccar GPS Tracking Server
   traccar:
-    image: traccar/traccar:latest
+    image: traccar/traccar:6.11
     container_name: vip-traccar
     ports:
       - "8082:8082"   # Web UI & API