Re-wrap prose once during pane motion

Hold the document at its narrower width while a pane track animates so
prose re-wraps once rather than every frame, and defer workspace
re-measurement until the sidebar settles. Collapse the closing Chat
pane's borders so it ends flush, and keep keyboard sidebar resizing
instant.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>