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/settings.py

@@ -398,10 +398,18 @@ def get_settings_sections(config: dict, threshold_checker=None) -> list:
         {
             "id": "dns",
             "title": "Dynamic DNS",
-            "description": "nsupdate-based DNS registration — edit raw YAML.",
-            "section_mode": "yaml",
+            "description": "nsupdate-based DNS registration via nsupdate(8).",
+            "section_mode": "form",
             "api_section": "dns",
-            "fields": [],
+            "fields": [
+                field("nsupdate_bin", "nsupdate binary", "path",
+                      "Path to the nsupdate binary.", editable=True),
+                field("rndc_key", "RNDC key file", "path",
+                      "Path to the rndc key file used to authenticate DNS updates.", editable=True),
+                field("dyndomains", "Dynamic domains", "list",
+                      "Domains updated via nsupdate when a host with dyndns: true reports in.",
+                      editable=True),
+            ],
         },
         {
             "id": "users",