[Feature] Add live Markdown preview panel

[kom50]

Summary

Show rendered Markdown preview alongside the editor.

Requirements

• Split view: editor | preview
• Real-time preview updates
• Synchronized scrolling
• Toggle preview on/off
• Support GFM (GitHub Flavored Markdown)
• Syntax highlighting in code blocks
• Only show when language is Markdown

UI/UX

• Preview panel on right (50% width)
• Toggle button in header
• Styled like GitHub Markdown
• Smooth transitions

Technical Notes

• Use marked or markdown-it library
• Use highlight.js for code block highlighting

<script src="https://cdn.jsdelivr.net/npm/marked/marked.min.js"></script>

Acceptance Criteria

• Markdown renders correctly in preview
• Preview updates in real-time as user types
• Code blocks have syntax highlighting
• Scrolling is synchronized between panels