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/http.py

@@ -1304,9 +1304,15 @@ async def start(
                             attrs.pop("client_secret", None)
                 data["oauth"] = new_oauth
 
-            for section in ("notification_channels", "dns"):
-                if section in payload:
-                    configio_mod.apply_yaml_section(data, section, payload[section])
+            if "notification_channels" in payload:
+                configio_mod.apply_yaml_section(data, "notification_channels", payload["notification_channels"])
+
+            if "dns" in payload:
+                dns_payload = payload["dns"]
+                if isinstance(dns_payload, str):
+                    configio_mod.apply_yaml_section(data, "dns", dns_payload)
+                else:
+                    configio_mod.apply_structured_section(data, "dns", dns_payload)
 
             if "thresholds" in payload:
                 tc = payload["thresholds"]