feat: add Pagefind client-side search

Add Pagefind indexing after each Eleventy build with a search page at
/search/. Indexes main content only (sidebars excluded), supports dark
mode theming and URL query parameters (?q=).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package.json

@@ -19,7 +19,8 @@
     "@quasibit/eleventy-plugin-sitemap": "^2.2.0",
     "@atproto/api": "^0.12.0",
     "eleventy-plugin-embed-everything": "^1.21.0",
-    "rss-parser": "^3.13.0"
+    "rss-parser": "^3.13.0",
+    "pagefind": "^1.3.0"
   },
   "devDependencies": {
     "tailwindcss": "^3.4.0",