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
- 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
- Integrated BookmarksService to display and filter bookmarked notes in quick links and paginated list
- Added bookmark toggle button in note viewer with visual feedback (filled/outline icon states)
- Implemented accordion behavior in mobile sidebar where opening one section closes others
- Updated responsive breakpoints to treat tablet as mobile (desktop layout starts at 1024px)
