feat: replace Dynamic DNS YAML editor with a web form

Adds structured form fields for nsupdate_bin, rndc_key, and dyndomains
(comma-separated list). Wires list-type editable fields through the
generic stageFormSection path and adds DNS support to
apply_structured_section in configio.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hbd/server/templates/settings.html

@@ -820,6 +820,11 @@
                     <input type="number" class="field-input"
                            data-key="{{ f.key }}" data-type="{{ f.type }}" data-section="{{ section.api_section }}"
                            value="{{ f.raw if f.raw is not none else '' }}">
+                  {% elif f.type == 'list' %}
+                    <input type="text" class="field-input"
+                           data-key="{{ f.key }}" data-type="list" data-section="{{ section.api_section }}"
+                           value="{{ f.value | join(', ') if f.value else '' }}"
+                           placeholder="comma-separated">
                   {% else %}
                     <input type="text" class="field-input"
                            data-key="{{ f.key }}" data-section="{{ section.api_section }}"
@@ -1023,6 +1028,8 @@
           } else if (el.dataset.type === 'number' || el.dataset.type === 'port') {
             const v = parseInt(el.value, 10);
             _staged[apiSection][key] = isNaN(v) ? null : v;
+          } else if (el.dataset.type === 'list') {
+            _staged[apiSection][key] = el.value.split(',').map(s => s.trim()).filter(Boolean);
           } else {
             _staged[apiSection][key] = el.value;
           }