Upgrade prettier from 2 to 3

[tykeal]

Problem

prettier is at 2.8.2; latest is 3.x. The pre-commit config pins the
prettier mirror to v2.5.1, so the dependency and the hook should move
together.

Proposed change

Upgrade prettier to ^3, bump the mirrors-prettier rev in
.pre-commit-config.yaml to the matching v3 release, run
prettier --write across the tracked file set, and commit the
(potentially large) reformat as its own atomic commit.

Acceptance criteria

• prettier on ^3 and the pre-commit mirror matches.
• Repo reformatted; npm run lint and pre-commit pass.