fix(cron): allow clearing accountId and treat undefined global mode as announce

- UI: always include accountId in patch so users can clear it
- delivery.ts: treat undefined global mode as announce when comparing for clearing inherited 'to'

src/cron/delivery.ts

@@ -171,7 +171,9 @@ export function resolveFailureDestination(
       // Mode was explicitly overridden - clear inherited 'to' since URL semantics differ
       // between announce (channel recipient) and webhook (HTTP endpoint)
       // But preserve explicit 'to' that was set at job level
-      if (!jobToExplicit && globalConfig?.mode !== jobMode) {
+      // Treat undefined global mode as "announce" for comparison
+      const globalMode = globalConfig?.mode ?? "announce";
+      if (!jobToExplicit && globalMode !== jobMode) {
         to = undefined;
       }
       mode = jobMode;

ui/src/ui/controllers/cron.ts

@@ -606,12 +606,13 @@ function buildFailureAlert(form: CronFormState) {
     channel: form.failureAlertChannel.trim() || CRON_CHANNEL_LAST,
     to: form.failureAlertTo.trim() || undefined,
     ...(cooldownMs !== undefined ? { cooldownMs } : {}),
-    ...(accountId ? { accountId } : {}),
   };
-  // Always include mode so users can switch between webhook/announce
+  // Always include mode and accountId so users can switch/clear them
   if (deliveryMode) {
     patch.mode = deliveryMode;
   }
+  // Include accountId if explicitly set, or send undefined to allow clearing
+  patch.accountId = accountId || undefined;
   return patch;
 }