onboarding: continuous donut animation, horizontal yes/no keys, auto-open model picker

- Keep the idle donut animating during the onboarding welcome screen instead
  of only repainting on input events (idle_donut_active + deep-idle guards now
  treat onboarding_welcome_active as needing the animation loop).
- Import walkthrough Yes/No sit side-by-side, so movement keys now match:
  Left/h -> Yes, Right/l -> No, Up/Down/k/j/Tab toggle; y/n/Enter/Space commit.
- ModelSelect now auto-opens the model picker so options render immediately;
  copy guides the user to browse and pick (or run /model) rather than blindly
  pressing Enter.