test: add Jest configuration and fix test path resolution

- Add jest.config.js with proper Node.js environment configuration
- Fix train.test.js to use correct relative path: '../../js/train.js'
- Add module.exports to train.js for Node.js module loading
- Update package.json with proper test scripts (test:unit, test:e2e, test:coverage)

jest.config.js

@@ -0,0 +1,13 @@
+module.exports = {
+    testEnvironment: 'node',
+    testMatch: ['**/test/**/*.test.js'],
+    verbose: true,
+    clearMocks: true,
+    resetModules: true,
+    coverageReporters: ['text', 'lcov', 'html'],
+    collectCoverageFrom: [
+        'js/**/*.js',
+        '!js/**/__tests__/**',
+        '!js/**/*.test.js'
+    ]
+};