feat: add bookmark view modes (card/tile/cover) and enhance button block with visual config modal
- Added three view modes for bookmark blocks: card (default with side image), tile (compact horizontal), and cover (large top image)
- Implemented view mode submenu in block context menu with visual indicators for active mode
- Enhanced bookmark favicon handling with fallback logic and error state tracking
- Refactored button block with inline visual editor replacing text inputs
- Created Button
- Integrated Unsplash API for image search functionality with environment configuration
- Added new Nimbus Editor page component with navigation from sidebar and mobile drawer
- Enhanced TOC with highlight animation for editor heading navigation
- Improved CDK overlay z-index hierarchy for proper menu layering
- Removed obsolete logging validation script
