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/author/term.html

@@ -1,6 +1,6 @@
 {{ define "title" }}Posts by {{ .Title }} · {{ site.Title }}{{ end }}
 {{ define "main" }}
-<p class="crumbs">{{ with site.GetPage "/post" }}<a href="{{ .RelPermalink }}">{{ .Title }}</a>{{ end }} › <a href="{{ .Parent.RelPermalink }}">Authors</a></p>
+<p class="crumbs" data-pagefind-ignore>{{ with site.GetPage "/post" }}<a href="{{ .RelPermalink }}">{{ .Title }}</a>{{ end }} › <a href="{{ .Parent.RelPermalink }}">Authors</a></p>
 <h1>Posts by {{ .Title }}</h1>
 <div class="posts">
   {{ range .Pages.ByDate.Reverse }}{{ partial "post-summary.html" . }}{{ end }}