⬆️(frontend) upgrade frontend to ESLint 9

Manually upgrade the frontend codebase to ESLint 9.

Update the linting configuration and related code where required to
maintain compatibility with the new major version.

src/frontend/eslint.config.js

@@ -0,0 +1,78 @@
+// src/frontend/eslint.config.js
+import js from '@eslint/js'
+import tseslint from 'typescript-eslint'
+import reactPlugin from 'eslint-plugin-react'
+import reactHooks from 'eslint-plugin-react-hooks'
+import reactRefresh from 'eslint-plugin-react-refresh'
+import jsxA11y from 'eslint-plugin-jsx-a11y'
+import globals from 'globals'
+
+export default tseslint.config(
+  // Global ignores (replaces .eslintignore)
+  {
+    ignores: [
+      'dist/**',
+      'node_modules/**',
+      'coverage/**',
+      'src/styled-system/**',
+    ],
+  },
+
+  // Base JS recommended rules
+  js.configs.recommended,
+
+  // TypeScript recommended rules (parser + rules bundled together)
+  ...tseslint.configs.recommended,
+
+  // React-specific config
+  {
+    files: ['**/*.{ts,tsx}'],
+    plugins: {
+      react: reactPlugin,
+      'react-hooks': reactHooks,
+      'react-refresh': reactRefresh,
+      'jsx-a11y': jsxA11y,
+    },
+    languageOptions: {
+      globals: {
+        ...globals.browser,
+      },
+      parserOptions: {
+        ecmaFeatures: { jsx: true },
+      },
+    },
+    settings: {
+      react: { version: 'detect' },
+    },
+    rules: {
+      // React rules
+      ...reactPlugin.configs.recommended.rules,
+      ...reactHooks.configs.recommended.rules,
+      'react/react-in-jsx-scope': 'off', // not needed with React 17+
+      'react-refresh/only-export-components': [
+        'warn',
+        { allowConstantExport: true },
+      ],
+
+      // jsx-a11y recommended rules
+      ...jsxA11y.configs.recommended.rules,
+
+      // TypeScript rules you may want to adjust
+      '@typescript-eslint/no-unused-vars': [
+        'error',
+        { argsIgnorePattern: '^_' },
+      ],
+      '@typescript-eslint/no-explicit-any': 'warn',
+    },
+  },
+
+  // CJS and root-level JS config files (postcss.config.js, etc.)
+  {
+    files: ['**/*.cjs', '*.config.js'],
+    languageOptions: {
+      globals: {
+        ...globals.node,
+      },
+    },
+  }
+)