Migrate ESLint configuration to v9 flat config format

[Copilot]

ESLint v9 drops support for .eslintrc.* files and requires the new flat config format.

Changes

• Add eslint.config.mjs - Migrates all existing rules from .eslintrc.js to flat config format
  • Preserves TypeScript parser configuration and plugin rules
  • Maintains ignore patterns and custom rule overrides
  • Uses .mjs extension to avoid Node.js module resolution warnings

Configuration Example

export default [
  {
    ignores: ['node_modules/', 'build/', 'dist/', '*.js', '!eslint.config.mjs'],
  },
  {
    files: ['**/*.ts', '**/*.tsx'],
    languageOptions: {
      parser: tsParser,
      parserOptions: { ecmaVersion: 2018, sourceType: 'module' },
    },
    plugins: { '@typescript-eslint': typescriptEslint },
    rules: {
      ...typescriptEslint.configs.recommended.rules,
      '@typescript-eslint/no-var-requires': 'off',
      // ... other custom rules
    },
  },
];

The old .eslintrc.js can be removed once this is merged.

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