Add site search (Pagefind)

Client-side index generated after the Hugo build; search box in the
header, results page with a section filter.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YDRjtmAiNpo5mK5N3d7QwZ

layouts/post/single.html

@@ -1,11 +1,11 @@
 {{ define "main" }}
 <article class="post">
-  <p class="crumbs"><a href="{{ .Parent.RelPermalink }}">{{ .Parent.Title }}</a></p>
+  <p class="crumbs" data-pagefind-ignore><a href="{{ .Parent.RelPermalink }}">{{ .Parent.Title }}</a></p>
   <h1>{{ .Title }}</h1>
   <p class="meta">{{ if not .Date.IsZero }}<time datetime="{{ .Date.Format "2006-01-02" }}">{{ .Date.Format "2 January 2006" }}</time>{{ end }}{{ with .GetTerms "author" }}{{ if not $.Date.IsZero }} · {{ end }}{{ partial "terms.html" (dict "page" $ "taxonomy" "author") }}{{ end }}</p>
   {{ with partial "banner.html" (dict "page" . "width" 1000) }}<img class="post-banner" src="{{ . }}" alt="">{{ end }}
   <div class="content">{{ .Content }}</div>
-  <nav class="prev-next">
+  <nav class="prev-next" data-pagefind-ignore>
     {{ with .PrevInSection }}<a href="{{ .RelPermalink }}">← {{ .Title }}</a>{{ end }}
     {{ with .NextInSection }}<a href="{{ .RelPermalink }}">{{ .Title }} →</a>{{ end }}
   </nav>