Weight Pagefind search results by read count

## Goal

When two posts match a query about equally on text relevance, prefer the one more readers have engaged with. Small extension on top of Pagefind (already shipped with AstroPaper).

## Implementation sketch

- Pagefind supports custom [sort](https://pagefind.app/docs/sorts/) and [filter](https://pagefind.app/docs/filtering/) attributes via \`data-pagefind-sort\` HTML attributes.
- At build time, after generating read-count JSON (#142), inject \`data-pagefind-sort=\"reads:\"\` on each post's wrapper element.
- Search UI exposes an optional \"sort by popularity\" toggle, or blends popularity into the default rank with a low weight.

## Why low priority

- Personal blog corpus is small; Pagefind's text relevance is already good enough most queries.
- Real value only kicks in once the corpus grows past ~50 posts.

## Depends on

- #141 (analytics source)
- #142 (per-post read-count data must already be baked into the build)