Add src/lib/date.ts (formatPostDate) and use it in BlogSidebar (was raw
{post.date}) plus the post header and archive cards, replacing the two inline
fmtDate copies. All blog dates now render "Month DD, YYYY" consistently.
Bump 0.1.12 -> 0.1.13.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>