Design: the subscriptions page's headroom uses the design system, and headroom leaves §5d #3834

For: the design audit (#2678): one page at a time onto §5's components, so docs/DESIGN.md §5d only shrinks. Headroom is how the owner steers spend.

Done:

• [ ] The subscriptions page's headroom figures is drawn from §5's components (src/web/ui.ts helpers such as row, card, badge, btn, and §5 classes) in place of .headroom, which src/web/style/content.ts defines today.
• [ ] Those classes' rules are gone, and their names leave their bullet of docs/DESIGN.md §5d. bun scripts/design-lint.ts passes, and so does --update with a lower or equal baseline.
• [ ] The pull carries before/after screenshots in light and dark, at desktop and 390 px wide. Nothing a person reads or taps is lost, and every form posts the same fields.
• [ ] The page's existing tests pass. A test asserts the old class names are absent from the page's HTML.

Files: src/web/pages/subscriptions.ts, src/web/style/content.ts, docs/DESIGN.md, the page's test. Risk: another page borrowing one of these classes. git grep each name first, and if one is used elsewhere, keep it and say so in the pull.

Parked by the crew: the check failed in the place it ran, not in the change, and failed the same way when it was run again (pull #3859 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Steward (trevato): pull #3859 was parked by an infrastructure fault, not this work - the egress cap from #3719 stopped every check's package install (02:20-03:12Z). The forge is rolled back and the cap is fixed (c7ea1935). Reopened and re-checked; this strike does not count.

Parked by the crew: the check stayed red through 2 automatic tries (pull #3859 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Parked by the crew: the run ended without a pull: clone failed: fatal: unable to parse commit 1446f69e8ea54fbdf88845347be4133484fd163e warning: Clone succeeded, but checkout failed. You can inspect what was checked out with 'git status' and retry with 'git restore

Parked twice since a person last spoke here, so it is the planner's now. Every reason:

1. the check stayed red through 2 automatic tries (pull #3859 closed)
2. the run ended without a pull: clone failed: fatal: unable to parse commit 1446f69e8ea54fbdf88845347be4133484fd163e warning: Clone succeeded, but checkout failed. You can inspect what was checked out with 'git status' and retry with 'git restore

The planner splits it, replaces it with a fresh issue, or defers it; a person can also say what to try. Nothing starts here until then, and the board lists it.

Split into #4404 (the headroom forms' markup, keeping the narrow-screen wrap) and #4405 (the rules and the DESIGN.md name). The first try stayed red doing both plus screenshots; the second died on a broken clone. Screenshots are dropped because the sandbox has no browser.