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/auth-actions.ts

@@ -12,10 +12,19 @@ const registerSchema = z.object({
   name: z.string().optional(),
 })
 
+const loginSchema = z.object({
+  email: z.string().email({ message: 'Invalid email address' }),
+  password: z.string().min(1, { message: 'Password is required' }),
+})
+
 export type RegisterResult = 
   | { success: true; userId: string }
   | { success: false; error: string }
 
+export type LoginResult = 
+  | { success: true }
+  | { success: false; error: string }
+
 /**
  * Register a new user with email and password
  * Requirements: 1.1, 1.2, 1.3, 1.4, 1.5
@@ -81,3 +90,45 @@ export async function registerUser(
     return { success: false, error: 'Registration failed. Please try again.' }
   }
 }
+
+/**
+ * Login user with email and password
+ * Requirements: 2.1, 2.2
+ */
+export async function loginUser(
+  email: string,
+  password: string
+): Promise<LoginResult> {
+  try {
+    // Validate input (Requirement 2.1 - credential validation)
+    const validation = loginSchema.safeParse({ email, password })
+    
+    if (!validation.success) {
+      const errors = validation.error.issues.map((issue) => issue.message).join(', ')
+      return { success: false, error: errors }
+    }
+
+    const { email: validEmail, password: validPassword } = validation.data
+
+    // Attempt to sign in (Requirement 2.1 - authenticate and create session)
+    // The authorize function in auth.ts handles:
+    // - Querying user from database
+    // - Password verification with bcrypt
+    // - Session creation on success
+    await signIn('credentials', {
+      email: validEmail,
+      password: validPassword,
+      redirect: false,
+    })
+
+    // If signIn doesn't throw an error, authentication was successful
+    return { success: true }
+  } catch (error) {
+    // Handle invalid credentials (Requirement 2.2)
+    // NextAuth throws an error for invalid credentials
+    console.error('Login error:', error)
+    
+    // Return generic error message to avoid revealing whether email exists
+    return { success: false, error: 'Invalid email or password' }
+  }
+}