Add bookmark and read later

People open a few posts at once and never come back. Let's give them a way to save stuff.

How I think it should work:
- bookmark button on each post card and on src/pages/blog/[slug].astro near the title (similar placement as RecentlyViewedPosts)
- store bookmarks in localStorage, same pattern as src/hooks/useRecentlyViewed.ts — no login for v1
- add /bookmarks page that lists saved posts with remove and clear all
- show count badge in header
- add "export as markdown" for the reading list so people can take it with them

Edge cases: handle posts that get renamed or deleted, and don't break if localStorage is full or disabled.