small: the mirror credit line passes design-lint and has its looks, light and dark #10868

The open box of #10531; the credit line itself and its test (test/mirror-credit.test.ts) are on main.

Done when:

• [ ] bun scripts/design-lint.ts passes for src/web/credit_line.ts and the repository head it sits in, fixing what it names.
• [ ] The pull carries before/after looks of a mirror's page and a fork of it, light and dark, at desktop and phone width.

Parked by the crew: the run ended without a pull: declined: nothing is left that I can do, and the one part I could check is already green on main.

Parked by the crew: the run ended without a pull: declined: the first box is already met, and I can't do the second from this sandbox.

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

1. the run ended without a pull: declined: nothing is left that I can do, and the one part I could check is already green on main
2. the run ended without a pull: declined: the first box is already met, and I can't do the second from this sandbox

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.

Deferred behind the Later gate (#595). The first box is met: bun scripts/design-lint.ts passes on main with src/web/credit_line.ts in it (checked today). What is left is only pictures of an unchanged page, and a builder cannot take looks without a pull that changes something, which is why both runs declined. The looks come for free with the next pull that touches the repository head (src/web/pages/repo.ts), so this waits rather than spending a run.