feat: add file block with drag-and-drop support
- Implemented file upload block with preview support for images, videos, PDFs, text, code, and DOCX files
- Added drag-and-drop directive for files with visual drop indicators in root and column contexts
- Created file icon component with FontAwesome integration for 50+ file type icons
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- 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
- Replaced generic alignment buttons with custom image-specific icons (left/center/right/full width)
- Added "Default size" button to reset image dimensions and aspect ratio
- Implemented modal dialogs for image info and caption editing to replace browser prompts
- Enhanced lateral drop detection to create 2-column layouts when dragging blocks side-by-side
- 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
