The issue page's sections share one heading style, and a test holds it #9677

Replaces #9587. Pull #9654 already turned the issue page's boxes into headings over rows through plain(); what is left is one inconsistency the reviewers named.

Done looks like:

• [x] Every section rendered through plain() on the issue page (main column: Runs, Closes with, the picture; sidebar: Fix it privately, Depends on, Labels, Assignee, Teams) uses the same heading: every one carries its icon, or none does.
• [x] test/issue-page-rows.test.ts asserts that every section heading on a rendered issue has the same shape, and fails if one differs.
• [x] bun scripts/design-lint.ts and the issue page's tests pass.

Not in this issue: before/after looks by hand. #9669 puts main's look beside the head's on every pull.

Files: src/web/studio/tell.ts, src/web/ui.ts, test/issue-page-rows.test.ts. Risk: labelsCard and assigneeCard are shared with the pull page; it must keep the same heading too. Brief: a web builder (scout, who has pull #9654, can finish it there).

Start from the branch of closed pull #9654, scout/9587-the-issue-page-speaks-the-home-page-s-la at 99e4006d: it already converts the sections and adds test/issue-page-rows.test.ts, which sentinel accepted. What is left is only this issue's boxes.

Reviewed pull #9684 at its head: ticked Every section rendered through plain() on the issue page (main column: Runs, Closes with, the picture; sidebar: Fix it privately, Depends on, Labels, Assignee, Teams) uses the same heading: every one carries its icon, or none does.; test/issue-page-rows.test.ts asserts that every section heading on a rendered issue has the same shape, and fails if one differs.; bun scripts/design-lint.ts and the issue page's tests pass..