Implement authentication functionality with login server action

- Created loginUser server action with credential validation
- Implemented password verification using bcrypt (via NextAuth authorize)
- Session creation on successful login handled by NextAuth
- Invalid credentials handled with appropriate error messages
- Added comprehensive unit tests for login functionality
- All tests passing (11/11)

Requirements: 2.1, 2.2

lib/__tests__/auth-actions.test.ts

@@ -14,7 +14,8 @@ vi.mock('@/auth', () => ({
 }))
 
 // Import after mocking
-import { registerUser } from '../auth-actions'
+import { registerUser, loginUser } from '../auth-actions'
+import { signIn } from '@/auth'
 
 describe('registerUser', () => {
   // Clean up test users after each test
@@ -125,3 +126,97 @@ describe('registerUser', () => {
     expect(user?.name).toBeNull()
   })
 })
+
+describe('loginUser', () => {
+  // Clean up test users after each test
+  afterEach(async () => {
+    await prisma.user.deleteMany({
+      where: {
+        email: {
+          contains: 'test-login-',
+        },
+      },
+    })
+    vi.clearAllMocks()
+  })
+
+  it('should successfully login with valid credentials', async () => {
+    const email = `test-login-${Date.now()}@example.com`
+    const password = 'password123'
+
+    // Create a test user first
+    await registerUser(email, password, 'Test User')
+
+    // Mock signIn to succeed
+    vi.mocked(signIn).mockResolvedValueOnce(undefined as any)
+
+    // Attempt login
+    const result = await loginUser(email, password)
+
+    expect(result.success).toBe(true)
+    expect(signIn).toHaveBeenCalledWith('credentials', {
+      email,
+      password,
+      redirect: false,
+    })
+  })
+
+  it('should reject login with invalid credentials', async () => {
+    const email = `test-login-${Date.now()}@example.com`
+    const password = 'password123'
+
+    // Create a test user first
+    await registerUser(email, password, 'Test User')
+
+    // Mock signIn to throw error (invalid credentials)
+    vi.mocked(signIn).mockRejectedValueOnce(new Error('Invalid credentials'))
+
+    // Attempt login with wrong password
+    const result = await loginUser(email, 'wrongpassword')
+
+    expect(result.success).toBe(false)
+    if (!result.success) {
+      expect(result.error).toContain('Invalid email or password')
+    }
+  })
+
+  it('should reject login with non-existent email', async () => {
+    const email = `test-login-nonexistent-${Date.now()}@example.com`
+    const password = 'password123'
+
+    // Mock signIn to throw error (user not found)
+    vi.mocked(signIn).mockRejectedValueOnce(new Error('User not found'))
+
+    // Attempt login with non-existent email
+    const result = await loginUser(email, password)
+
+    expect(result.success).toBe(false)
+    if (!result.success) {
+      expect(result.error).toContain('Invalid email or password')
+    }
+  })
+
+  it('should reject login with invalid email format', async () => {
+    const email = 'invalid-email'
+    const password = 'password123'
+
+    const result = await loginUser(email, password)
+
+    expect(result.success).toBe(false)
+    if (!result.success) {
+      expect(result.error).toContain('Invalid email')
+    }
+  })
+
+  it('should reject login with empty password', async () => {
+    const email = `test-login-${Date.now()}@example.com`
+    const password = ''
+
+    const result = await loginUser(email, password)
+
+    expect(result.success).toBe(false)
+    if (!result.success) {
+      expect(result.error).toContain('Password is required')
+    }
+  })
+})