feat(posts): search, upcoming events, RSS feed

- Client-side search on /posts/ (full archive, ~2-char threshold)
- Upcoming-events block at top of /posts/ from isEvent+eventDate posts
- RSS feed at /posts/rss.xml via @astrojs/rss
- Layout head: rss autodiscovery link
- RSS subscribe link below the listing
- Style tweaks on QuoteBlock and PostOverviewBlock

package.json

@@ -17,6 +17,7 @@
     "firebase-deploy:nocache": "yarn build && firebase deploy --only hosting"
   },
   "dependencies": {
+    "@astrojs/rss": "^4.0.18",
     "@astrojs/sitemap": "^3.7.2",
     "@astrojs/svelte": "^8.0.4",
     "@fontsource-variable/lora": "^5.2.8",