instead of each shimmer needing to have its own animation thread, have
render_ref schedule a new frame if it wants one and coalesce to the
earliest next frame. this also makes the animations
frame-timing-independent, based on start time instead of frame count.
This sets up the scaffolding and basic flow for a TUI onboarding
experience. It covers sign in with ChatGPT, env auth, as well as some
safety guidance.
Next up:
1. Replace the git warning screen
2. Use this to configure default approval/sandbox modes
Note the shimmer flashes are from me slicing the video, not jank.
https://github.com/user-attachments/assets/0fbe3479-fdde-41f3-87fb-a7a83ab895b8
