Fix NaN propagation in alert grouping window and AI duration display

setGroupingWindow stored Number.parseInt result without checking for
NaN, while the adjacent setEscalationAfter properly guards against it.
If an invalid value reached the parser, NaN would propagate into the
grouping config and could cascade into alert delivery issues.

formatDuration in AIModelSelectionSection displayed raw NaN/Infinity/
negative values without guarding. Now returns '-' for non-finite or
negative inputs.

frontend-modern/src/components/Settings/AIModelSelectionSection.tsx

@@ -126,7 +126,8 @@ export const PatrolPreflightControl: Component<{ state: AISettingsState }> = (co
   };
 
   const formatDuration = (ms: number) => {
-    if (ms < 1000) return `${ms}ms`;
+    if (!Number.isFinite(ms) || ms < 0) return '-';
+    if (ms < 1000) return `${Math.round(ms)}ms`;
     return `${(ms / 1000).toFixed(1)}s`;
   };
 

frontend-modern/src/features/alerts/useAlertScheduleState.ts

@@ -254,9 +254,10 @@ export function useAlertScheduleState(props: UseAlertScheduleStateProps) {
   };
 
   const setGroupingWindow = (rawValue: string) => {
+    const parsed = Number.parseInt(rawValue, 10);
     props.setGrouping({
       ...props.grouping(),
-      window: Number.parseInt(rawValue, 10),
+      window: Number.isNaN(parsed) ? props.grouping().window : parsed,
     });
     markUnsaved();
   };