chore: upgrade ESLint 8→9 with flat config migration

- eslint 8.57.1 to 9.39.4
- @typescript-eslint/eslint-plugin 7.18.0 to 8.59.4
- @typescript-eslint/parser 7.18.0 to 8.59.4
- globals 11.12.0 to 17.6.0
- @eslint/eslintrc and @eslint/js added for FlatCompat
- Replace .eslintrc.cjs + .eslintignore with eslint.config.mjs
- Use flat configs for react, react-hooks, typescript-eslint directly
- FlatCompat only for airbnb-base (no flat config support yet)
- Fix no-unused-vars override from airbnb and react/display-name: off

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

.eslintignore

@@ -1,2 +0,0 @@
-experiment
-node_modules

.eslintrc.cjs

@@ -1,75 +0,0 @@
-module.exports = {
-  env: {
-    browser: true,
-    es2021: true,
-  },
-  extends: [
-    'eslint:recommended',
-    'plugin:react/recommended',
-    'plugin:react-hooks/recommended',
-    'plugin:@typescript-eslint/eslint-recommended',
-    'plugin:@typescript-eslint/recommended',
-    'airbnb',
-    'prettier',
-  ],
-  parser: '@typescript-eslint/parser',
-  parserOptions: {
-    ecmaFeatures: {
-      jsx: true,
-    },
-    ecmaVersion: 'latest',
-    sourceType: 'module',
-  },
-  globals: {
-    JSX: 'readonly',
-  },
-  plugins: ['react', '@typescript-eslint'],
-  rules: {
-    'linebreak-style': 0,
-    'no-underscore-dangle': 0,
-    'no-shadow': 'off',
-
-    'import/prefer-default-export': 'off',
-    'import/extensions': 'off',
-    'import/no-unresolved': 'off',
-    'import/no-extraneous-dependencies': [
-      'error',
-      {
-        devDependencies: true,
-      },
-    ],
-
-    'react/no-unstable-nested-components': ['error', { allowAsProps: true }],
-    'react/jsx-filename-extension': [
-      'error',
-      {
-        extensions: ['.tsx', '.jsx'],
-      },
-    ],
-
-    'react/require-default-props': 'off',
-    'react/jsx-props-no-spreading': 'off',
-    'react-hooks/rules-of-hooks': 'error',
-    'react-hooks/exhaustive-deps': 'error',
-    // React Compiler rules added in react-hooks v7 — disabled until React Compiler is adopted
-    'react-hooks/react-compiler': 'off',
-    'react-hooks/incompatible-library': 'off',
-    'react-hooks/refs': 'off',
-    'react-hooks/set-state-in-effect': 'off',
-    'react-hooks/set-state-in-render': 'off',
-    'react-hooks/immutability': 'off',
-    'react-hooks/purity': 'off',
-    'react-hooks/use-memo': 'off',
-
-    '@typescript-eslint/no-unused-vars': 'error',
-    '@typescript-eslint/no-shadow': 'error',
-  },
-  overrides: [
-    {
-      files: ['*.ts'],
-      rules: {
-        'no-undef': 'off',
-      },
-    },
-  ],
-};

eslint.config.mjs

@@ -0,0 +1,99 @@
+import { FlatCompat } from '@eslint/eslintrc';
+import js from '@eslint/js';
+import tsPlugin from '@typescript-eslint/eslint-plugin';
+import tsParser from '@typescript-eslint/parser';
+import reactPlugin from 'eslint-plugin-react';
+import reactHooksPlugin from 'eslint-plugin-react-hooks';
+import eslintConfigPrettier from 'eslint-config-prettier';
+import globals from 'globals';
+import path from 'path';
+import { fileURLToPath } from 'url';
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+
+const compat = new FlatCompat({
+  baseDirectory: __dirname,
+  recommendedConfig: js.configs.recommended,
+  allConfig: js.configs.all,
+});
+
+export default [
+  { ignores: ['node_modules/**', 'dist/**', 'experiment/**'] },
+  js.configs.recommended,
+  tsPlugin.configs['flat/eslint-recommended'],
+  ...tsPlugin.configs['flat/recommended'],
+  reactPlugin.configs.flat.recommended,
+  reactHooksPlugin.configs.flat['recommended'],
+  // airbnb-base via FlatCompat (JS/import rules; no React plugin, no getFilename issue)
+  ...compat.extends('airbnb-base'),
+  eslintConfigPrettier,
+  {
+    languageOptions: {
+      parser: tsParser,
+      globals: {
+        ...globals.browser,
+        ...globals.es2021,
+        JSX: 'readonly',
+      },
+      parserOptions: {
+        ecmaFeatures: { jsx: true },
+        ecmaVersion: 'latest',
+        sourceType: 'module',
+      },
+    },
+    settings: {
+      react: {
+        version: '18.2.0',
+      },
+    },
+    rules: {
+      'linebreak-style': 0,
+      'no-unused-vars': 'off', // handled by @typescript-eslint/no-unused-vars
+      'no-underscore-dangle': 0,
+      'no-shadow': 'off',
+
+      'import/prefer-default-export': 'off',
+      'import/extensions': 'off',
+      'import/no-unresolved': 'off',
+      'import/no-extraneous-dependencies': [
+        'error',
+        {
+          devDependencies: true,
+        },
+      ],
+
+      'react/no-unstable-nested-components': ['error', { allowAsProps: true }],
+      'react/jsx-filename-extension': [
+        'error',
+        {
+          extensions: ['.tsx', '.jsx'],
+        },
+      ],
+
+      'react/display-name': 'off',
+      'react/require-default-props': 'off',
+      'react/jsx-props-no-spreading': 'off',
+      'react-hooks/rules-of-hooks': 'error',
+      'react-hooks/exhaustive-deps': 'error',
+      // React Compiler rules added in react-hooks v7 — disabled until React Compiler is adopted
+      'react-hooks/react-compiler': 'off',
+      'react-hooks/incompatible-library': 'off',
+      'react-hooks/refs': 'off',
+      'react-hooks/set-state-in-effect': 'off',
+      'react-hooks/set-state-in-render': 'off',
+      'react-hooks/immutability': 'off',
+      'react-hooks/purity': 'off',
+      'react-hooks/use-memo': 'off',
+
+      '@typescript-eslint/no-unused-vars': 'error',
+      '@typescript-eslint/no-shadow': 'error',
+    },
+  },
+  {
+    files: ['**/*.ts'],
+    rules: {
+      'no-undef': 'off',
+    },
+  },
+];

package.json

@@ -66,6 +66,8 @@
     "@atlaskit/pragmatic-drag-and-drop": "1.8.1",
     "@atlaskit/pragmatic-drag-and-drop-auto-scroll": "2.1.5",
     "@atlaskit/pragmatic-drag-and-drop-hitbox": "1.1.0",
+    "@eslint/eslintrc": "3.3.5",
+    "@eslint/js": "10.0.1",
     "@fontsource-variable/inter": "5.2.8",
     "@giphy/js-fetch-api": "5.8.0",
     "@giphy/js-types": "5.1.0",
@@ -91,6 +93,7 @@
     "file-saver": "2.0.5",
     "focus-trap-react": "12.0.2",
     "folds": "2.6.2",
+    "globals": "17.6.0",
     "html-dom-parser": "7.1.0",
     "html-react-parser": "6.1.2",
     "i18next": "26.2.0",
@@ -143,15 +146,15 @@
     "@types/react-google-recaptcha": "2.1.9",
     "@types/sanitize-html": "2.16.1",
     "@types/ua-parser-js": "0.7.39",
-    "@typescript-eslint/eslint-plugin": "7.18.0",
-    "@typescript-eslint/parser": "7.18.0",
+    "@typescript-eslint/eslint-plugin": "8.59.4",
+    "@typescript-eslint/parser": "8.59.4",
     "@vanilla-extract/css": "1.20.1",
     "@vanilla-extract/recipes": "0.5.7",
     "@vanilla-extract/vite-plugin": "5.2.2",
     "@vitejs/plugin-react": "6.0.2",
     "buffer": "6.0.3",
     "cz-conventional-changelog": "3.3.0",
-    "eslint": "8.57.1",
+    "eslint": "9.39.4",
     "eslint-config-airbnb": "19.0.4",
     "eslint-config-prettier": "10.1.8",
     "eslint-plugin-import": "2.32.0",