feat: sort bar on papers site — top hits, newly updated, recently added
Added pill-style sort buttons above the papers grid. Default sort is "top hits" (by view count from paper-hit API). Also sorts by "newly updated" (last build time) and "recently added" (creation date). Papers now carry data-created and data-updated attributes for client-side sorting.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>