chore: upgrade ESLint from 8.x to 9.x+

[hb2708]

Summary

Upgrade eslint and its ecosystem from 8.x to 9.x+. This is the biggest tooling migration as ESLint 9+ requires the new flat config format (eslint.config.js), replacing the legacy .eslintrc format.

Current → Target

Package	Current	Latest
eslint	8.57.1	10.0.2
eslint-config-prettier	8.5.0	10.1.8
eslint-plugin-prettier	4.2.1	5.5.5
eslint-plugin-jest	27.1.3	29.15.0
eslint-plugin-react-hooks	4.6.0	7.0.1
eslint-import-resolver-typescript	3.5.2	4.4.4
@typescript-eslint/eslint-plugin	^8.49.0	latest
@typescript-eslint/parser	^8.49.0	latest

Why Upgrade

• ESLint 8.x is end-of-life
• Flat config is the future — all new plugins target it
• Better performance and simpler configuration
• eslint-plugin-react-hooks 7.x has significant improvements

Migration Steps

1. Create eslint.config.js (flat config) to replace existing ESLint config:

   # Use the migration helper
   npx @eslint/migrate-config .eslintrc.json

2. Install updated packages:

   yarn add --dev eslint@latest \
     eslint-config-prettier@latest \
     eslint-plugin-prettier@latest \
     eslint-plugin-jest@latest \
     eslint-plugin-react@latest \
     eslint-plugin-react-hooks@latest \
     eslint-import-resolver-typescript@latest \
     eslint-plugin-import@latest \
     @typescript-eslint/eslint-plugin@latest \
     @typescript-eslint/parser@latest

3. Convert config — flat config uses JS objects instead of JSON:
   • extends → ...configs (spread)
   • plugins → import and reference directly
   • env → languageOptions.globals
   • parserOptions → languageOptions.parserOptions
4. Update lint scripts in package.json — flat config auto-discovers eslint.config.js
5. Remove old config files — delete .eslintrc, .eslintrc.json, or .eslintrc.js
6. Fix any new lint errors introduced by updated rule defaults
7. Run yarn ci to verify

⚠️ Recommendation

This should be done after the Prettier 3.x upgrade (issue for that exists separately #431 ), since eslint-plugin-prettier and eslint-config-prettier need to be compatible with both ESLint and Prettier versions.

Files Affected

• .eslintrc* → eslint.config.js (new file, old file deleted)
• package.json — dependency versions, possibly lint script changes
• Potentially some source files if new/stricter rules flag issues

References

• ESLint Flat Config Migration Guide
• typescript-eslint v8 Guide
• eslint-plugin-react-hooks v7