feat: redesign code block header with professional action buttons and tabbed menu
- Replaced collapsible header with dedicated action buttons for line numbers, word wrap, and collapse/preview
- Added custom SVG icons for each action button with active/inactive states and hover effects
- Implemented tabbed menu interface with Language/Theme/Font/Options sections replacing vertical scrolling
- Added collapsed preview mode with syntax highlighting using cached highlightedHtml
- Introduced auto-detect language toggle
