fix(session): add keyboard support to question dock

[nexxeln]

Summary

• make the session question dock keyboard-native with answer focus, arrow navigation, Escape dismiss, and Mod+Enter next/submit
• keep custom answer editing and multi-select transitions consistent when moving between typed and built-in answers
• add app e2e coverage for keyboard submit and dismiss flows

Testing

• bun typecheck (packages/app)
• bun typecheck (packages/ui)