Fix a UI bug with Design Mode

Design Mode collapses the "that button looks wrong" → "fixed commit" loop to under a minute.

Steps

1. Open the worktree's browser pane. Navigate to the page with the bug.
2. Toggle Design Mode on.
3. Click the broken element. It lands in the agent chat as a rich attachment.
4. Type what you want fixed: "this padding is too tight, increase to match the cards above."
5. The agent edits the source. Hot reload refreshes the browser.
6. Click the element again to verify — if still wrong, repeat.
7. When it's right, commit.

Why it's fast

No screenshot, no DOM hunting, no selector copying. The agent gets the HTML, computed CSS, and a cropped image of the exact element you pointed at — the same context a human reviewer would want.