Add Edge Broker service and configuration defaults

Introduced the Edge Broker service in the Docker Compose setup, configured to run on port 4300. Updated `.env.example` to include default environment variables for the Edge Broker and adjusted tests to validate the new configuration. Updated visual snapshot tests to reflect related UI changes.

docker-compose.example.yml

@@ -46,6 +46,16 @@ services:
       timeout: 5s
       retries: 10
 
+  edge-broker:
+    build:
+      context: .
+      dockerfile: services/edge-broker/Dockerfile
+    container_name: edge-broker
+    environment:
+      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET:-truckwash-edge-dev}
+    ports:
+      - "4300:4300"
+
   caddy:
     image: caddy:2.7.6-alpine
     container_name: caddy