Restyle the decision card to the approved design

Drop the Decision label row for a mark beside the question, lettered option rows
with a trailing check, an always-last Add an option row, Discard/Save footer
with in-place confirm, a stepper in place of question tabs, and a shared
callout for a failed save. Adds DecisionIcon.

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