feat: implement route protection middleware for authentication

- Created Next.js middleware for authentication checks
- Protected main routes (/, /list/*, /profile/*, /invitations/*)
- Redirect unauthenticated users to login with callback URL
- Handle session validation and expiration
- Allow access to auth pages for unauthenticated users
- Redirect authenticated users away from login/register pages
- Added comprehensive middleware tests
- Validates Requirements 3.3, 3.5

Task: 7. Implement route protection middleware

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

@@ -83,14 +83,14 @@
   - **Property 12: Session duration persistence**
   - **Validates: Requirements 3.4**
 
-- [ ] 6. Create authentication UI components
+- [x] 6. Create authentication UI components
   - Build LoginForm component with email/password inputs
   - Build RegisterForm component with validation
   - Create authentication error display page
   - Add loading states and error handling
   - _Requirements: 1.1, 2.1_
 
-- [ ] 7. Implement route protection middleware
+- [x] 7. Implement route protection middleware
   - Create Next.js middleware for authentication checks
   - Protect main routes (/, /list/*, /profile/*)
   - Redirect unauthenticated users to login