Add ask my blog chat widget (RAG over blog content)

I write long technical posts and people often want the short version or to find the one paragraph that answers their question.

Try a small RAG widget:
- at build time, chunk content/blog markdown and generate embeddings (OpenAI or local), store in a simple JSON or Turso
- at runtime, a floating button opens a chat panel that searches Pagefind + vectors, answers with citations linking back to the post and heading
- limit scope to only my content, add a disclaimer that it can be wrong
- reuse the existing shiki theme and UI style, don't bring in a huge chat framework
- gate it behind an env flag so I can turn it off if costs get weird

Start with 20-30 most recent posts to keep embedding costs low. If it works, expand to journal and projects. This is also a good demo piece for the portfolio.