Add adaptive animated chat composer

[jasonhedman]

Summary

• keep short chat drafts and send controls on one compact row
• expand the composer when text wraps, includes a newline, or adds content
• animate composer height with Motion while respecting reduced-motion preferences
• anchor the compact submit button to the bottom-right so it has no horizontal layout projection
• add position-only scale correction for both the submit button and compact textarea content
• enable the compact composer in both new-chat and existing-session inputs

Testing

• pnpm check (60 test files, 253 tests)
• production build with documented local environment placeholders
• focused Motion-contract regressions for compact submit and textarea behavior
• independent post-remediation review: clean