Prettier streaming TUI

[ivy]

Inspiration

• A demo video for Plandex: https://player.vimeo.com/video/926634577
  • 00:34, there's a loading indicator before the LLM response begins streaming.
• aichat has a common indicator that I've found appealing.
• Fira Code (and probably other fonts) supports ligatures for progress spinners. It looks pretty slick!