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/single.html

@@ -1,6 +1,6 @@
 {{ define "main" }}
 <article>
-  {{ with .Parent }}{{ if not .IsHome }}<p class="crumbs"><a href="{{ .RelPermalink }}">{{ .Title }}</a>{{ with $.Params.group }} › {{ . }}{{ end }}</p>{{ end }}{{ end }}
+  {{ with .Parent }}{{ if not .IsHome }}<p class="crumbs" data-pagefind-ignore><a href="{{ .RelPermalink }}">{{ .Title }}</a>{{ with $.Params.group }} › {{ . }}{{ end }}</p>{{ end }}{{ end }}
   <h1>{{ .Title }}</h1>
   <div class="content">{{ .Content }}</div>
 </article>