feat: Add comprehensive input validation

- Created lib/validation.ts with validation utilities for all form inputs
- Added validation for list titles, item titles, colors, dates, tags, descriptions, and notes
- Implemented inline validation with real-time error feedback in ListForm
- Enhanced ItemForm with comprehensive validation for all fields
- Updated InlineItemCreate with validation
- Added input sanitization to prevent XSS attacks
- Updated server actions to use validation utilities
- Added maxLength attributes to prevent excessive input
- Improved accessibility with aria-invalid and aria-describedby attributes
- Added visual feedback for validation errors with red borders
- Validates Requirements 1.1 and 2.1

.kiro/specs/ultimate-lists-app/tasks.md

@@ -199,7 +199,7 @@
   - Implement skeleton loaders
   - _Requirements: 7.3_
 
-- [-] 16. Implement error handling
+- [x] 16. Implement error handling
   - Add try-catch blocks to all server actions
   - Implement error boundaries in React components
   - Add user-friendly error messages
@@ -207,7 +207,7 @@
   - Implement retry logic for failed operations
   - _Requirements: All (error handling is cross-cutting)_
 
-- [ ] 17. Add input validation
+- [x] 17. Add input validation
   - Validate all form inputs before submission
   - Add inline validation errors
   - Prevent empty task/list creation