Add support for blog series and learning paths

I've got a few posts that build on each other (Cookie Store, Durable Streams, Speech Synthesis) but there's no way to read them as a sequence.

Plan:
- add optional `series: string` and `seriesOrder: number` to src/content.config.js for blogPosts
- create src/pages/blog/series/[series].astro listing posts in order with progress (e.g. 2 of 4)
- show a small banner on the post page when a post is part of a series: prev/next links + progress bar
- update SimilarPosts.astro to prioritize same-series posts first, then tag overlap fallback
- add a SeriesBadge component similar to TagBadge.astro

No need for complex drag-and-drop ordering. Just a string field and a number. Should be easy to query with getCollection.