Feature: collapse/expand cards

Would be nice to have a little widget on the left that could expand/collapse a card, even on cards that don't have a header.

The issue names no files, tests, or entry points. Start by locating the card-rendering UI and any existing header controls, then determine how cards without headers are represented. Done should include a left-side expand/collapse control that works for every card and preserves the intended card content.

Written by the indexing model from the issue text.