feat: add collapsible block type and enhance paragraph menu UX
- Added CollapsibleBlockComponent with registration in block host and columns
- Enhanced paragraph block with inline "+" button for column layouts and improved menu positioning
- Updated toggle block to support creating new toggle blocks below with dedicated event handlers
- Improved menu styling with fixed positioning, keyboard navigation (arrows, Enter, Escape), and focus management
- Simplified inline action handling by deleg
- Replaced emoji icons with consistent SVG icons for checkbox, bullet, and numbered lists in context menu and inline toolbar
- Enhanced drag-drop visual feedback with subtle highlight on target blocks during dragging
- Refined column creation logic to use deterministic drop modes (column-left, column-right, column-gap) for more predictable behavior
- Improved comment button z-index to prevent overlap with drag indicators
- 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
