Implement session management (Task 5)

- Configure JWT session strategy with 30-day expiration
- Implement session validation middleware for route protection
- Create logout functionality with session termination
- Handle session expiration automatically via JWT
- Add session helper functions (getCurrentSession, isAuthenticated)
- Protect routes: /, /list/*, /profile/*, /invitations/*
- Redirect unauthenticated users to login with callback URL
- Requirements: 2.4, 2.5, 3.1, 3.2, 3.4

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

@@ -56,7 +56,7 @@
   - **Property 7: Invalid credentials rejection**
   - **Validates: Requirements 2.2**
 
-- [ ] 5. Implement session management
+- [x] 5. Implement session management
   - Configure JWT session strategy in NextAuth
   - Implement session validation middleware
   - Create logout functionality