Replace ESLint with Biome

[dwjohnston]

Replaces the ESLint setup with Biome for linting.

Changes:

• Remove eslint and eslint-config-next devDependencies
• Add @biomejs/biome ^1.9.4 as devDependency
• Create biome.json with recommended rules, a11y, and formatter config
• Update lint script from eslint . to biome check .
• Remove .eslintrc.json

Closes #267

Generated with Claude Code

[dwjohnston]

@claude - This appears to be checking a bunch of generated folders. Sort out the ignore list please.

[claude]

Claude finished @dwjohnston's task in 2m 17s —— View job

---

Fix Biome ignore list

• Read current biome.json
• Identify generated folders in the project
• Update biome.json ignore list
• Commit and push

Changes made

Updated biome.json with two improvements:

1. Added VCS integration — tells Biome to respect .gitignore files, which automatically excludes:

• .cache/
• .netlify/
• /public/build
• .env files
• And anything else already in .gitignore

2. Added explicit ignores for generated files not covered by .gitignore:

• next-env.d.ts — auto-generated by Next.js
• tsconfig.tsbuildinfo — TypeScript incremental build output

"vcs": {
  "enabled": true,
  "clientKind": "git",
  "useIgnoreFile": true
},

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[dwjohnston]

@claude there are ~60 errors. Fix them.

[claude]

Fix Biome Errors

• Gather context
• Identify errors (found ~30+ manual-fix issues across multiple categories)
• Fix useButtonType errors (~22 buttons missing type attribute)
• Fix noExplicitAny errors (2 occurrences)
• Fix noArrayIndexKey errors (1 occurrence)
• Fix unused import errors (~5 occurrences)
• Fix accessibility issues (~2 occurrences)
• Commit and push

View job run

[dwjohnston]

@claude sorry - I hadn't pushed a commit. Check again

[claude]

Claude finished @dwjohnston's task in 9m 1s —— View job

---

Fix Biome Errors

• Gather context — CI failed due to network error (ECONNRESET during npm i), not biome errors. User's biome --write --unsafe commits are already on the branch.
• Run Biome to identify errors — found remaining non-auto-fixable errors: a11y/useButtonType on 37 <button> elements across 12 files
• Fix all errors — added type="button" to all buttons without an explicit type
• Commit and push

Summary

The biome check --write --unsafe run (from your prior commits) fixed all auto-fixable issues. The remaining errors were a11y/useButtonType — Biome requires <button> elements to have an explicit type attribute, but this rule is not auto-fixable, so it needs manual fixes.

Fixed 12 files with 37 button elements across:

• src/app/game-of-life/page.tsx
• src/demos/cache/components/RequestRequiresValidation.tsx
• src/demos/imperative_confirmation_modal/hooks/useConfirmationModal.tsx
• src/demos/react-renders/ReactRenders.tsx, ReactRenders2.tsx, ReactRenders3.tsx, ReactRenders3b.tsx, ReactRenders3c.tsx, ReactRenders5.tsx
• src/demos/responsive_cookies/components/UserProfile1.tsx, UserProfile2.tsx, UserProfile3.tsx

Note: The console.log statements in these demo files are not flagged by Biome's recommended rules, so those were left as-is.