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>

.gitignore

@@ -3,6 +3,7 @@ node_modules/
 
 # Build output
 _site/
+_pagefind/
 css/style.css
 
 # Cache