Update chatStream method to use real Tauri event-based streaming:
- Add StreamChatEvent types matching Rust backend
- Set up Tauri event listener for 'stream:chunk' events
- Route events to appropriate callbacks (onDelta, onTool, onComplete, onError)
- Clean up listener on completion or error
- Remove simulated streaming fallback
This completes the frontend streaming integration for Chunk 4.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>