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

.kiro/specs/user-auth-and-sharing/tasks.md

@@ -6,7 +6,7 @@
   - Set up environment variables for auth secrets
   - _Requirements: 1.1, 2.1_
 
-- [ ] 2. Update database schema for multi-user support
+- [x] 2. Update database schema for multi-user support
   - Add User model with email, passwordHash, and name fields
   - Add Collaborator model for list sharing
   - Add Invitation model for sharing invitations
@@ -26,7 +26,7 @@
   - **Property 3: Password hashing on registration**
   - **Validates: Requirements 1.3**
 
-- [ ] 3. Implement user registration functionality
+- [x] 3. Implement user registration functionality
   - Create registration server action with email/password validation
   - Implement password hashing with bcrypt
   - Add email uniqueness check