Introduce formatBlogDate, slugifyTag and getTagHref in src/lib/blog.ts
and
a reading time helper getReadingTime in src/lib/readingTime.ts. Update
blog index and post pages to display ISO dates, reading times and link
tags to /blog/tag/<slug>. Add a tag listing page at
src/pages/blog/tag/[tag].astro with styles and static paths generation
