v0.5.0: Fetch Kafka consumer groups on demand

Remove consumerGroups from TopicDetails and add a new Tauri command kafka_topic_consumer_groups that scans groups and fetches committed offsets per-partition (sorted by group and partition). Register the new command in main.rs. Update frontend: add kafkaApi.topicConsumerGroups, change TopicView to mount ConsumersTab lazily, and rewrite ConsumersTab to fetch groups on mount with loading/error UI and manual Refresh. This keeps topic open cheap and performs the expensive group scan only when the Consumers tab is opened.