Add reactions and live presence on blog posts

ViewCounter works but it only counts. Would be nice to see if anyone else is actually reading right now.

Extend src/pages/api/views/[slug].ts (currently Redis) to also handle reactions:
- four reactions max: fire, lightbulb, heart, hands-clap — keep it simple, no custom emoji picker
- one reaction per person per post (store a flag in localStorage, check server side too)
- show totals under the post, next to GiscusWrapper
- add "2 people reading now" using a short TTL key in Redis, increment on page view, expire after 2 minutes

Don't build a full comment-like system. Just counts. Rate limit the endpoint so someone can't spam it.