feat: replace YAML hosts editor with form-based CRUD table

Settings > Hosts now renders a table with per-column controls
(watch, dyndns, owner, managers/monitors multi-select, threshold
config, notification channels) instead of a raw YAML textarea.
Changes stage via the existing Publish flow like other form sections.

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

hbd/server/http.py

@@ -1049,6 +1049,8 @@ async def start(
             title="Settings - Heartbeat",
             sections=settings_data["sections"],
             all_channel_names=settings_data["all_channel_names"],
+            all_usernames=settings_data["all_usernames"],
+            all_threshold_configs=settings_data["all_threshold_configs"],
             current_user=current_user.to_dict() if current_user else None,
             active_page="settings",
         )
@@ -1226,10 +1228,17 @@ async def start(
                             attrs.pop("client_secret", None)
                 data["oauth"] = new_oauth
 
-            for section in ("notification_channels", "thresholds", "hosts", "dns"):
+            for section in ("notification_channels", "thresholds", "dns"):
                 if section in payload:
                     configio_mod.apply_yaml_section(data, section, payload[section])
 
+            if "hosts" in payload:
+                h = payload["hosts"]
+                if isinstance(h, dict):
+                    configio_mod.apply_structured_section(data, "hosts", h)
+                else:
+                    configio_mod.apply_yaml_section(data, "hosts", h)
+
             configio_mod.write_config(_config_path, data)
         except Exception as exc:
             logger.error("Config write failed: %s", exc)