chore(lint): migrate ESLint 8 to 9 with flat config

Replace legacy .eslintrc.js with eslint.config.mjs (flat config).
Upgrade eslint ^8.45 -> ^9.27, @typescript-eslint/* ^6.2 -> ^8.32,
add @eslint/js. Disable no-undef (TypeScript handles this).

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

Author: vveerrgg

.eslintrc.js

@@ -0,0 +1,39 @@
+import eslint from '@eslint/js';
+import tseslint from '@typescript-eslint/eslint-plugin';
+import tsparser from '@typescript-eslint/parser';
+
+export default [
+  {
+    ignores: ['dist/**', 'node_modules/**', 'coverage/**', 'examples/**'],
+  },
+  {
+    files: ['**/*.ts'],
+    languageOptions: {
+      parser: tsparser,
+      parserOptions: {
+        ecmaVersion: 'latest',
+        sourceType: 'module',
+      },
+      globals: {
+        console: 'readonly',
+        process: 'readonly',
+      },
+    },
+    plugins: {
+      '@typescript-eslint': tseslint,
+    },
+    rules: {
+      ...eslint.configs.recommended.rules,
+      ...tseslint.configs.recommended.rules,
+      'no-undef': 'off',
+      '@typescript-eslint/no-explicit-any': 'off',
+      '@typescript-eslint/explicit-function-return-type': 'off',
+      '@typescript-eslint/explicit-module-boundary-types': 'off',
+      '@typescript-eslint/no-unused-vars': ['error', {
+        argsIgnorePattern: '^_',
+        varsIgnorePattern: '^_',
+        caughtErrorsIgnorePattern: '^_',
+      }],
+    },
+  },
+];

eslint.config.mjs

@@ -13,8 +13,8 @@
         "test:coverage": "vitest run --coverage",
         "test:ui": "vitest --ui",
         "dev": "tsc -w",
-        "lint": "eslint . --ext .ts",
-        "lint:fix": "eslint . --ext .ts --fix",
+        "lint": "eslint .",
+        "lint:fix": "eslint . --fix",
         "prepublishOnly": "npm run lint && npm run test && npm run build"
     },
     "main": "dist/nodes/index.js",
@@ -60,11 +60,12 @@
         "@types/node": "^20.19.33",
         "@types/node-red": "^1.3.5",
         "@types/ws": "^8.18.1",
-        "@typescript-eslint/eslint-plugin": "^6.2.0",
-        "@typescript-eslint/parser": "^6.2.0",
+        "@eslint/js": "^9.27.0",
+        "@typescript-eslint/eslint-plugin": "^8.32.1",
+        "@typescript-eslint/parser": "^8.32.1",
         "@vitest/coverage-v8": "^3.2.4",
         "@vitest/ui": "^3.2.4",
-        "eslint": "^8.45.0",
+        "eslint": "^9.27.0",
         "node-red": "^4.1.5",
         "node-red-node-test-helper": "^0.3.6",
         "typescript": "^5.9.3",