feat: implement user registration functionality

- Created registerUser server action with email/password validation
- Implemented password hashing with bcrypt (10 rounds)
- Added email uniqueness check to prevent duplicate registrations
- Created session after successful registration using NextAuth
- Added comprehensive validation for email format and password length (min 8 chars)
- Added email and password validation utilities to lib/validation.ts
- Removed PrismaAdapter from auth.ts (using JWT sessions only)
- Added vitest testing framework and configuration
- Created comprehensive unit tests for registration functionality
- All tests passing (6/6)

Requirements covered: 1.1, 1.2, 1.3, 1.4, 1.5

lib/validation.ts

@@ -301,3 +301,46 @@ export function validateItemData(data: {
     },
   }
 }
+
+/**
+ * Validate email format
+ */
+export function validateEmail(email: string): ValidationResult {
+  const sanitized = sanitizeString(email)
+  
+  if (!sanitized) {
+    return { isValid: false, error: 'Email is required' }
+  }
+  
+  // Basic email regex pattern
+  const emailRegex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/
+  
+  if (!emailRegex.test(sanitized)) {
+    return { isValid: false, error: 'Invalid email format' }
+  }
+  
+  if (sanitized.length > 255) {
+    return { isValid: false, error: 'Email must be less than 255 characters' }
+  }
+  
+  return { isValid: true }
+}
+
+/**
+ * Validate password strength
+ */
+export function validatePassword(password: string): ValidationResult {
+  if (!password) {
+    return { isValid: false, error: 'Password is required' }
+  }
+  
+  if (password.length < 8) {
+    return { isValid: false, error: 'Password must be at least 8 characters' }
+  }
+  
+  if (password.length > 128) {
+    return { isValid: false, error: 'Password must be less than 128 characters' }
+  }
+  
+  return { isValid: true }
+}