chore(deps-dev): upgrade ESLint 8 → 9.x with flat config migration (#356)

* chore(deps-dev): upgrade eslint to 9.x with flat config migration

- Upgrade eslint 8.57.1 → 9.39.4
- Migrate .eslintrc (legacy) to eslint.config.js (flat config)
- Replace abandoned eslint-config-react-app with @eslint/js, typescript-eslint,
  eslint-plugin-react-hooks, and globals
- Add lint script to package.json and lint step to CI
- Fix @ts-ignore → @ts-expect-error and remove unused variables surfaced by new rules

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* chore(deps-dev): remove eslint-plugin-react-hooks (YAGNI)

src/ has no hooks; only test fixtures use them.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* chore(eslint): remove dead rules (YAGNI)

Drop rules already covered by js.configs.recommended, disabled by
eslint-config-prettier, or unreachable due to no-var.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* ci: trigger pipeline check

* chore(eslint): fix globals — vitest not jest, drop es2015

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* ci: use npm install instead of npm ci

rolldown (vitest 4.x) has platform-specific optional binaries. A lockfile
generated on macOS omits linux variants; npm ci then fails on ubuntu runners
with 'Cannot find @rolldown/binding-linux-x64-gnu'. npm install resolves
the correct platform binaries regardless of what the lockfile recorded.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Revert "ci: use npm install instead of npm ci"

This reverts commit 02e82f3.

* fix(deps): regenerate lockfile with all-platform optional deps

The lockfile regenerated during the rebase on macOS dropped the linux
rolldown bindings (npm/cli#4828), breaking npm ci on ubuntu runners.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: arturosdg

.eslintignore

@@ -17,6 +17,7 @@ jobs:
           node-version-file: .nvmrc
       - run: |
           npm ci
+          npm run lint
           npm run test
           npm run build
           npm run check:exports

.eslintrc

@@ -0,0 +1,52 @@
+import js from '@eslint/js'
+import globals from 'globals'
+import reactPlugin from 'eslint-plugin-react'
+import prettierRecommended from 'eslint-plugin-prettier/recommended'
+import tseslint from 'typescript-eslint'
+
+export default tseslint.config(
+  { ignores: ['dist'] },
+  js.configs.recommended,
+  tseslint.configs.recommended,
+  reactPlugin.configs.flat.recommended,
+  {
+    languageOptions: {
+      globals: {
+        ...globals.node,
+        ...globals.vitest,
+      },
+      parserOptions: {
+        ecmaFeatures: { jsx: true },
+      },
+    },
+    settings: {
+      react: { version: 'detect' },
+    },
+    rules: {
+      'no-var': 'error',
+      eqeqeq: 'error',
+      'no-alert': 'error',
+      'no-eval': 'error',
+      'no-empty-function': 'error',
+      complexity: ['warn', 8],
+    },
+  },
+  {
+    files: ['**/*.ts', '**/*.tsx'],
+    rules: {
+      'no-empty-function': 'off',
+      '@typescript-eslint/no-explicit-any': 'off',
+      '@typescript-eslint/ban-ts-comment': [
+        'error',
+        { 'ts-expect-error': false, 'ts-ignore': true },
+      ],
+    },
+  },
+  {
+    files: ['**/*.d.ts'],
+    rules: {
+      '@typescript-eslint/no-empty-object-type': 'off',
+    },
+  },
+  prettierRecommended,
+)