Add grid layout at >= 640px breakpoint for #issue-list and #pull-list
containers, matching the existing .card-grid tablet behavior. Cards
render in a 2-column grid on tablet while maintaining single-column
on mobile.
Closesleeworks-agents/gitea-mobile#105
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>