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>

frontend/Dockerfile

@@ -46,7 +46,7 @@ EXPOSE 80
 
 # Health check
 HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
-  CMD wget --no-verbose --tries=1 --spider http://localhost/ || exit 1
+  CMD wget --no-verbose --tries=1 --spider http://127.0.0.1/ || exit 1
 
 # Start nginx
 CMD ["nginx", "-g", "daemon off;"]