v0.6.5: Add realtime Kafka consume/produce UI and backend

Introduce anonymous realtime Kafka consumer support and new UI surfaces. Backend: add KafkaConsumerRegistry and tauri commands kafka_consume_start / kafka_consume_stop that stream records (UTF‑8 + base64) via a Channel and manage per-partition poll tasks. Frontend: add ConsumeView, ProduceView, TopicListView, GroupListView, kafkaConsumerStore (batched external store), update types and useKafkaState (new view model + prefill tokens), refactor LeftPanel and TopicView to surface Consume/Produce flows, and integrate into KafkaExplorer (stop consumers on tool unmount). Also wire small RabbitMQ UI changes to open consumer detail panels. Add testing/kafka docker-compose and helpers to run a local Kafka broker, producer, and consumer for manual testing.