candidates: visible native checkboxes + tiny JS counter

- checkbox: form-check-input (Bootstrap) rendered invisible outside
  .form-check -> native checkbox with explicit width/height
- tiny JS: badge #sel-count updates on change of any #bulk-form checkbox
- base.html: added {% block scripts %}
- verified: render (200 checkboxes), JS counter logic (node), service restarted

web/templates/base.html

@@ -26,5 +26,6 @@
 <div class="container-fluid mt-2 px-3">
   {% block content %}{% endblock %}
 </div>
+{% block scripts %}{% endblock %}
 </body>
 </html>