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/configio.py

@@ -88,6 +88,12 @@ def apply_structured_section(data, section: str, values: dict) -> None:
         for key in _SERVER_KEYS:
             if key in values:
                 data[key] = values[key]
+    elif section == "dns":
+        for key in _DNS_KEYS:
+            if key in values:
+                data[key] = values[key]
+            else:
+                data.pop(key, None)
     elif section == "users":
         data["users"] = values
     elif section == "hosts":