feat: Redesign streaming UI with bottom-right messages and persistent popup

- Replace cascading message windows with debounced bottom-right streaming display
- Add top-right tool usage notifications with auto-hide
- Show final response in persistent popup (Esc/q to close)
- Implement 2-second debounce timer for streaming messages
- Streamline UI flow: thinking → streaming → tool usage → final popup

Author: avifenesh