.backups-drill, .backups-error, .backups-now, .backups-restore leave the stylesheet and docs/DESIGN.md §5d #4131

Part two of #3833. It starts once part one has removed these names from the markup.

Done:

• [ ] git grep each of .backups-drill .backups-error .backups-now .backups-restore: no use is left in src/ but its rule. If one is still used somewhere, keep it and say where in the pull.
• [ ] The rules leave src/web/style/content.ts (and its reduced-motion block), and the names leave their §5d bullet in docs/DESIGN.md.
• [ ] Run bun scripts/design-lint.ts --update and commit docs/design/baseline.json only if the count goes down or stays equal. bun test test/design-lint.test.ts passes.

Files: src/web/style/content.ts, docs/DESIGN.md, docs/design/baseline.json. Before pushing: bun x tsc --noEmit and bun test test/design-lint.test.ts. Rebase right before pushing: DESIGN.md changes under everyone.

Parked by the crew: the check failed ("a frozen mirror is said, and reconnected in one click > (unnamed)") and the decider saw no point in another try (pull #4350 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Parked by the crew: the check failed ("migrate: after > mirror: no write at migration, then one push to GitHub after a landing") and the decider saw no point in another try (pull #4443 closed).

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

1. the check failed ("a frozen mirror is said, and reconnected in one click > (unnamed)") and the decider saw no point in another try (pull #4350 closed)
2. the check failed ("migrate: after > mirror: no write at migration, then one push to GitHub after a landing") and the decider saw no point in another try (pull #4443 closed)

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.

Replaced by #4598. Both tries went red on tests the change never touched (migrate-after, origins-reconnect, livetriage, snapshot-mirror, strangers, or an "(unnamed)" hook timeout); those are now planned as #4577–#4582. The fresh issue keeps the diff to the named files and says to run design-lint and lower the baseline in the same pull.