fix: support list-valued threshold_config in hosts table

threshold_config in .hb.yaml can be a list (e.g. [local, zrepl]).
The hosts table was treating it as a single string, so the pre-selected
value never matched. Normalize to a list in settings.py, switch the
select to multiple, and fix the JS to collect all selected options.

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

hbd/server/settings.py

@@ -284,7 +284,10 @@ def get_settings_sections(config: dict, threshold_checker=None) -> list:
             "owner": hcfg.get("owner", ""),
             "managers": hcfg.get("managers", []),
             "monitors": hcfg.get("monitors", []),
-            "threshold_config": hcfg.get("threshold_config", ""),
+            "threshold_configs": (
+                list(v) if isinstance(v := hcfg.get("threshold_config"), list)
+                else ([v] if v else [])
+            ),
             "notification_channels": hcfg.get("notification_channels", []),
         })