fix(app): restore zen mode height animation

[ymichael]

Summary

• restore the zen-mode height animation effect by rerunning it when isZenMode changes
• guard the queued editor focus callback after zen toggles if the TipTap editor has already torn down
• add a regression test that verifies the prompt box applies the measured height transition

Regression

Introduced in b96107fa9 (TER-10: Add rich prompt editor mentions), where the textarea-specific animation dependencies were simplified and isZenMode was accidentally dropped.

Validation

• pnpm exec turbo run test --filter=@bb/app -- src/components/promptbox/PromptBoxInternal.test.tsx
• pnpm exec turbo run typecheck --filter=@bb/app