Markdown Rendering Improvements

Priority: P1 Issue: #6088

Headings h1-h6 render at the same visual size as body text. kilo-ui CSS only resets margins, no distinct font sizes/weights.

Remaining Work

• Add CSS rules for heading elements (h1-h6) with distinct font sizes, weights, and vertical spacing
• Scope heading styles to the markdown content area
• Verify styles across light, dark, and high-contrast VS Code themes
• Also verify: bullet lists, numbered lists, blockquotes, horizontal rules, and bold/italic