The dead `.headroom` rules leave src/web/style/content.ts and docs/DESIGN.md §5d, and no markup changes #4767

Intent: remove the last of .headroom. On main at 05d93e01 no markup draws it: the subscriptions page's headroom form is already .row-fields with a plain .input. Only dead rules remain.

Done:

• [ ] the .headroom block in src/web/style/content.ts is deleted (the comment, .headroom, .headroom .input and the two .row:has(.headroom) rules, about lines 216–222)
• [ ] .headroom leaves the content.ts list in docs/DESIGN.md §5d
• [ ] docs/design/baseline.json is lowered only where bun scripts/design-lint.ts reports a count fell, and is never raised
• [ ] src/web/pages/subscriptions.ts is not touched. A test asserts the rendered stylesheet has no .headroom rule and the subscriptions page still shows the headroom form (action=".../headroom", name="headroom")

Files: src/web/style/content.ts, docs/DESIGN.md, docs/design/baseline.json and one test. Nothing else.

Why it was parked: #4610 asked for the page to be redrawn as well. Its second pull went red on "design check … the tree is no worse than its baseline", because the redraw added markup outside §5. Nothing is left to redraw, so make no markup change. Run bun scripts/design-lint.ts and bun test test/design-lint.test.ts test/settings-visible.test.ts test/subscriptions.test.ts before pushing.