[mvantellingen]

Replace prettier and eslint with biome (formatter, import sorting and linter), modeled on the setup in labd/commercetools-node-mock.

• Add @biomejs/biome 2.5.15 and biome.jsonc. Formatting keeps the previous prettier settings (2 spaces, single quotes, semicolons, es5 trailing commas, 80 columns). jsxRuntime is set to reactClassic because the project uses "jsx": "react".
• Remove prettier, the prettier field in package.json and .prettierignore.
• Remove eslint (eslint, @typescript-eslint/parser, @typescript-eslint/eslint-plugin v5, .eslintrc.js). It only used eslint:recommended + @typescript-eslint/recommended, which biome's recommended rules cover.
• Scripts: lint = biome check, format = biome check --fix, new check = biome check && tsc --noEmit. The husky pre-commit hook still runs pnpm lint.
• CI runs pnpm biome ci.
• Formatting changes are in a separate style: format with biome commit.