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>

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",