#427 feat(desktop): add markdown syntax highlighting for Monaco editor

Summary

• Add token rules for markdown syntax highlighting in the Monaco diff viewer
• Headings and link text use blue, URLs use cyan, inline code uses green
• Bold, italic, and strikethrough get proper font styles

Test plan

• Open a markdown file diff in the desktop app
• Verify headings, links, code blocks, and formatting are highlighted

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Markdown syntax highlighting in the editor now properly applies custom theme colors to tokens, improving visual consistency with your selected theme.

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