Add an option appends a shared option for everyone

The add row is now a button that opens a focused field: Enter adds for the
whole room, Escape cancels, the new row shows dimmed while pending and a
rejection reopens the field with an error callout. Existing custom drafts
still render as a selected row.

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