search: replace Fuse.js with Pagefind (Default UI) + full UI polish

Remove the legacy Fuse.js search from html-head.html (it double-loaded
with the new Pagefind UI; backup kept at html-head.html.bkup) and add a
Pagefind Default-UI partial (search.html) injected into the sidebar via
menu.html.

Index scoping: tag the article element with data-pagefind-body in
baseof.html, term.html, and posts/{single,list}.html so Pagefind indexes
page content only (pages without the tag are skipped; 404.html stays
untagged).

search.html (all overrides #search-scoped, since pagefind-ui.css loads in
<body> after the head CSS):
  - compact input/magnifier/clear-button fitted to the sidebar; clear
    button uses fixed px (not %) because the results drawer is an in-flow
    child of the form, which stretches the form to results height
  - hide the empty .result-thumb placeholder (no images in the index) that
    was pushing result text ~90px right
  - restyle matched-term <mark> from UA yellow to subdued slate
  - keyboard navigation (Pagefind 1.5.2 has none; stock Enter is a silent
    no-op): capture-phase keydown implements ArrowUp/Down highlighting with
    wraparound across top-level hits AND their nested sub-results (each sub
    highlights its own .result-nested row), Enter opens the highlighted
    result (or the top result), and a fingerprint-aware MutationObserver
    keeps the highlight across Pagefind's async re-renders without
    resetting it on every DOM patch

themes/hugo-book/layouts/term.html

@@ -1,6 +1,6 @@
 {{ define "main" }}
   {{ range sort .Paginator.Pages }}
-  <article class="markdown book-post">
+  <article class="markdown book-post" data-pagefind-body>
     <h2>
       <a href="{{ .RelPermalink }}">{{ partial "docs/title" . }}</a>
     </h2>