chore: add prettier config and enforce formatting in CI

[Bilb]

Why

biome.json only includes **/*.ts and **/*.tsx, so json, css, md, yaml and js were formatted by nothing. And the Lint workflow ran biome lint, which checks lint rules only — formatting was never enforced anywhere.

This also fixes a real editor problem: with no .prettierrc in the repo, anyone whose editor formats on save with the Prettier extension gets Prettier's defaults, which disagree with biome.json in three places — printWidth 80 vs 100, trailingComma all vs es5, and embedded-CSS reformatting inside styled.* templates. On pages/pro.tsx that was a 345-line diff on every save.

Config

Prettier is pinned to match biome.json rather than compete with it:

{
  "singleQuote": true,
  "printWidth": 100,
  "trailingComma": "es5",
  "embeddedLanguageFormatting": "off"
}

Verified: with these options, prettier and biome format produce byte-identical output on all 109 ts/tsx files. The two tools cannot drift apart on the code they both cover.

.prettierignore excludes the lib/app_localization submodule, the generated contentful/export.json, and locales/ (Crowdin exports — they only differ by a missing trailing newline, and formatting them would fail CI on every sync).

Enforcement

package.json:

script	command
lint	biome lint && prettier --check .
lint:fix	biome lint --write && prettier --write .
format	prettier --write .
format:check	prettier --check .

The workflow runs the two tools as separate steps with if: always() on the format check, rather than a single pnpm run lint. This is deliberate — see below.

Note: this PR will show a red Lint step

biome lint already fails on main with 12 errors, and has since the workflow was added — run 33366786839 failed on the merge of #30, and the PR that introduced it failed too. The gate has never passed.

Those errors are pre-existing and unrelated to formatting (noExplicitAny across services/, noNonNullAssertion, noDangerouslySetInnerHtml, noExcessiveCognitiveComplexity, unreachable code in lib/proBackend.ts). They involve real code decisions, so I left them for a separate PR.

This is exactly why the workflow uses two steps: with a single biome lint && prettier --check ., the always-failing Biome step would short-circuit and the Prettier check would silently never run. Split, the Check formatting step passes while the Biome step stays red for the pre-existing reasons.

Diff

Formatting-only, no semantic changes. next.config.js is the bulk of it (quote normalisation + arrow-body wrapping); pages/[slug].tsx is a missing semicolon that Biome also flagged. Confirmed next.config.js, tailwind.config.js, postcss.config.js still require() to the same shape and tsc --showConfig still resolves 109 files.

[Bilb]

Superseded — Biome 2.1.2 formats JSON and CSS natively, so broadening its includes covers 121 of the 124 relevant files. Adding Prettier bought only README.md and two YAML files, at the cost of a second formatter config to keep in lockstep. Redoing this as a Biome-only change.