feat: add ESLint 9 configuration with TypeScript support

- Add eslint.config.js using new flat config format
- Configure @typescript-eslint/parser and plugin for TypeScript files
- Add separate config for vanilla JavaScript files (gateway/ui)
- Include Node.js and browser globals
- Enable strict rules: curly braces, no-eval, eqeqeq, etc.
- Configure TypeScript-specific rules (no-explicit-any, no-non-null-assertion)
- Add @typescript-eslint/parser and @typescript-eslint/eslint-plugin dependencies

eslint.config.js

@@ -0,0 +1,179 @@
+// ESLint 9+ flat config
+// TypeScript configuration
+
+import tsParser from '@typescript-eslint/parser';
+import tsPlugin from '@typescript-eslint/eslint-plugin';
+
+export default [
+  {
+    // TypeScript files
+    files: ['**/*.{ts,tsx}'],
+    languageOptions: {
+      parser: tsParser,
+      parserOptions: {
+        ecmaVersion: 2022,
+        sourceType: 'module',
+        project: './tsconfig.json',
+      },
+      globals: {
+        // Node.js globals
+        process: 'readonly',
+        console: 'readonly',
+        Buffer: 'readonly',
+        __dirname: 'readonly',
+        __filename: 'readonly',
+        global: 'readonly',
+        module: 'readonly',
+        require: 'readonly',
+        exports: 'writable',
+        // Common test globals
+        describe: 'readonly',
+        it: 'readonly',
+        test: 'readonly',
+        expect: 'readonly',
+        beforeEach: 'readonly',
+        afterEach: 'readonly',
+        beforeAll: 'readonly',
+        afterAll: 'readonly',
+        vi: 'readonly',
+      },
+    },
+    plugins: {
+      '@typescript-eslint': tsPlugin,
+    },
+    rules: {
+      // Possible errors
+      'no-console': 'off', // Allow console in Node.js
+      'no-debugger': 'warn',
+      'no-dupe-keys': 'error',
+      'no-duplicate-case': 'error',
+      'no-empty': ['error', { allowEmptyCatch: true }],
+      'no-ex-assign': 'error',
+      'no-extra-boolean-cast': 'error',
+      'no-func-assign': 'error',
+      'no-irregular-whitespace': 'error',
+      'no-unreachable': 'error',
+      'valid-typeof': 'error',
+
+      // Best practices
+      'curly': ['error', 'all'],
+      'eqeqeq': ['error', 'always', { null: 'ignore' }],
+      'no-eval': 'error',
+      'no-implied-eval': 'error',
+      'no-return-await': 'off', // Disabled in favor of TS version
+      '@typescript-eslint/return-await': 'error',
+      'no-throw-literal': 'error',
+      'no-unused-expressions': 'error',
+      'no-useless-concat': 'error',
+      'no-useless-return': 'error',
+      'prefer-promise-reject-errors': 'error',
+
+      // Variables - use TypeScript versions
+      'no-undef': 'off', // TypeScript handles this
+      'no-unused-vars': 'off', // Use TypeScript version
+      '@typescript-eslint/no-unused-vars': ['warn', {
+        argsIgnorePattern: '^_',
+        varsIgnorePattern: '^_',
+        caughtErrorsIgnorePattern: '^_',
+      }],
+
+      // TypeScript specific
+      '@typescript-eslint/no-explicit-any': 'warn',
+      '@typescript-eslint/no-non-null-assertion': 'warn',
+      '@typescript-eslint/explicit-function-return-type': 'off',
+      '@typescript-eslint/explicit-module-boundary-types': 'off',
+
+      // Stylistic (basic only, prefer Prettier for formatting)
+      'indent': ['error', 2, { SwitchCase: 1 }],
+      'quotes': ['error', 'single', { avoidEscape: true }],
+      'semi': ['error', 'always'],
+      'comma-dangle': ['error', 'always-multiline'],
+      'no-trailing-spaces': 'error',
+      'eol-last': ['error', 'always'],
+    },
+  },
+  {
+    // JavaScript files (no TypeScript parser)
+    files: ['**/*.{js,mjs,cjs}'],
+    languageOptions: {
+      ecmaVersion: 2022,
+      sourceType: 'module',
+      globals: {
+        // Node.js globals
+        process: 'readonly',
+        console: 'readonly',
+        Buffer: 'readonly',
+        __dirname: 'readonly',
+        __filename: 'readonly',
+        global: 'readonly',
+        module: 'readonly',
+        require: 'readonly',
+        exports: 'writable',
+        // Browser globals (for gateway/ui)
+        window: 'readonly',
+        document: 'readonly',
+        fetch: 'readonly',
+        WebSocket: 'readonly',
+        location: 'readonly',
+        URLSearchParams: 'readonly',
+        navigator: 'readonly',
+        alert: 'readonly',
+        confirm: 'readonly',
+        setTimeout: 'readonly',
+        setInterval: 'readonly',
+        clearTimeout: 'readonly',
+        clearInterval: 'readonly',
+      },
+    },
+    rules: {
+      // Possible errors
+      'no-console': 'off',
+      'no-debugger': 'warn',
+      'no-dupe-keys': 'error',
+      'no-duplicate-case': 'error',
+      'no-empty': ['error', { allowEmptyCatch: true }],
+      'no-ex-assign': 'error',
+      'no-extra-boolean-cast': 'error',
+      'no-func-assign': 'error',
+      'no-irregular-whitespace': 'error',
+      'no-unreachable': 'error',
+      'valid-typeof': 'error',
+
+      // Best practices
+      'curly': ['error', 'all'],
+      'eqeqeq': ['error', 'always', { null: 'ignore' }],
+      'no-eval': 'error',
+      'no-implied-eval': 'error',
+      'no-throw-literal': 'error',
+      'no-unused-expressions': 'error',
+      'no-useless-concat': 'error',
+      'no-useless-return': 'error',
+
+      // Variables
+      'no-undef': 'error',
+      'no-unused-vars': ['warn', {
+        argsIgnorePattern: '^_',
+        varsIgnorePattern: '^_',
+        caughtErrorsIgnorePattern: '^_',
+      }],
+
+      // Stylistic
+      'indent': ['error', 2, { SwitchCase: 1 }],
+      'quotes': ['error', 'single', { avoidEscape: true }],
+      'semi': ['error', 'always'],
+      'comma-dangle': ['error', 'always-multiline'],
+      'no-trailing-spaces': 'error',
+      'eol-last': ['error', 'always'],
+    },
+  },
+  {
+    // Ignore patterns
+    ignores: [
+      'dist/**',
+      'node_modules/**',
+      '**/*.d.ts',
+      'coverage/**',
+      '.pnpm-store/**',
+    ],
+  },
+];