feat: implement ERP authentication service

Implement ErpAuthService with TDD approach:
- Add login(), close(), getSession(), isActive() methods
- Use Playwright chromium with headless:false for debugging
- Manage browser lifecycle and session state
- Handle authentication flow with ERP_LOCATORS
- Add integration tests for login scenarios
- Add unit tests for session management
- Fix dotenv config path in test setup

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

tests/unit/erp-auth.unit.test.ts

@@ -0,0 +1,59 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+import { ErpAuthService } from '../../src/main/services/erp/erp-auth';
+import type { ErpConfig } from '../../src/main/types/erp.types';
+
+describe('ERP Authentication Service (Unit)', () => {
+  describe('Session Management', () => {
+    it('should create service instance with config', () => {
+      const config: ErpConfig = {
+        url: 'https://test.example.com',
+        username: 'testuser',
+        password: 'testpass',
+      };
+
+      const service = new ErpAuthService(config);
+
+      expect(service).toBeDefined();
+      expect(service.isActive()).toBe(false);
+    });
+
+    it('should throw error when getting session before login', () => {
+      const config: ErpConfig = {
+        url: 'https://test.example.com',
+        username: 'testuser',
+        password: 'testpass',
+      };
+
+      const service = new ErpAuthService(config);
+
+      expect(() => service.getSession()).toThrow('Not logged in. Call login() first.');
+    });
+
+    it('should report inactive status before login', () => {
+      const config: ErpConfig = {
+        url: 'https://test.example.com',
+        username: 'testuser',
+        password: 'testpass',
+      };
+
+      const service = new ErpAuthService(config);
+
+      expect(service.isActive()).toBe(false);
+    });
+  });
+
+  describe('Close Method', () => {
+    it('should handle close when no session exists', async () => {
+      const config: ErpConfig = {
+        url: 'https://test.example.com',
+        username: 'testuser',
+        password: 'testpass',
+      };
+
+      const service = new ErpAuthService(config);
+
+      // Should not throw when closing without session
+      await expect(service.close()).resolves.toBeUndefined();
+    });
+  });
+});