fix: correct plugin import path in docs, wire up orphaned footer template

docs/PLUGIN_DEVELOPMENT.md referenced a nonexistent hbd/plugin.py and
hbd/plugins/ directory; the real module is hbd/client/plugin.py with
plugins under hbd/client/plugins/, as README.md and the actual code
already use. Anyone following the doc would hit ModuleNotFoundError.

foot.html was never included by any template (confirmed via git grep
and template audit) despite being actively maintained; wired it into
about.html. menu.html was fully commented out and unreferenced, so
removed it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

hbd/server/templates/about.html

@@ -107,6 +107,8 @@
       </section>
     </div>
 
+    {% include 'foot.html' %}
+
     <script>
       (function() {
         var startEpoch = {{ start_epoch }};