Enhance PR/issue detail views with syntax highlighting and UI polish

[stylessh]

Summary

• Add shiki-based syntax highlighting to markdown code blocks with custom Vercel light/dark themes, per-block copy button, and React.use() + Suspense for flash-free rendering
• Move issue labels from main column to sidebar with theme-aware .label-pill styling (darker in light mode, vibrant in dark mode)
• Add icons to PR detail sidebar rows (calendar, clock, message, file, commit) and new CopyIcon/TickIcon exports
• Restructure PR header with copyable branch badges (<button> with tooltip on copy) and inline state badge + author
• Add diff boxes visualization to PR stats bar and shrink the +N -N text
• Darken green-500 in light mode only, switch to Geist Mono Variable font with font-weight: 450 and tighter letter-spacing for all monospace text

Test plan

• Verify syntax highlighting renders on PR/issue detail markdown code blocks (light + dark mode)
• Confirm no flash of unstyled code on page load or navigation
• Test copy button appears on hover over code blocks, copies content, shows checkmark
• Verify copyable branch badges in PR header work and truncate correctly for long names
• Check issue labels render in sidebar with correct colors in both themes
• Confirm diff boxes and stats display correctly in PR stats bar