ui(alerts): ReviewStep blocks save on empty webhooks+targets

Shows a warning banner and disables the Save button when a rule has
neither webhooks nor targets — would have been rejected at the server
edge (Task 3.3 validator), now caught earlier in the wizard with clear
reason.

ui/src/pages/Alerts/RuleEditor/ReviewStep.tsx

@@ -1,6 +1,26 @@
-import { Toggle } from '@cameleer/design-system';
+import { useMemo } from 'react';
+import { Alert, Toggle } from '@cameleer/design-system';
 import { toRequest, type FormState } from './form-state';
 
+/**
+ * Pure helper: returns a human-readable reason why saving should be blocked,
+ * or null when the rule is safe to save (from the wizard's perspective).
+ *
+ * Currently covers: a rule with no webhooks AND no targets would be rejected
+ * at the server edge (Task 3.3 validator) and would never notify anyone, so
+ * the wizard blocks it earlier with a clear reason. The helper is exported
+ * so `RuleEditorWizard` can also drive the Save button's `disabled` state
+ * off the same single source of truth.
+ */
+export function computeSaveBlockReason(form: FormState): string | null {
+  const noWebhooks = (form.webhooks ?? []).length === 0;
+  const noTargets = (form.targets ?? []).length === 0;
+  if (noWebhooks && noTargets) {
+    return 'Add at least one webhook or target \u2014 a rule with no recipients never notifies anyone.';
+  }
+  return null;
+}
+
 export function ReviewStep({
   form,
   setForm,
@@ -9,8 +29,14 @@ export function ReviewStep({
   setForm?: (f: FormState) => void;
 }) {
   const req = toRequest(form);
+  const saveBlockReason = useMemo(() => computeSaveBlockReason(form), [form]);
   return (
     <div style={{ display: 'grid', gap: 12, maxWidth: 720 }}>
+      {saveBlockReason && (
+        <Alert variant="error" title="Rule cannot be saved yet">
+          {saveBlockReason}
+        </Alert>
+      )}
       <div>
         <strong>Name:</strong> {form.name}
       </div>

ui/src/pages/Alerts/RuleEditor/RuleEditorWizard.tsx

@@ -19,7 +19,7 @@ import { ScopeStep } from './ScopeStep';
 import { ConditionStep } from './ConditionStep';
 import { TriggerStep } from './TriggerStep';
 import { NotifyStep } from './NotifyStep';
-import { ReviewStep } from './ReviewStep';
+import { ReviewStep, computeSaveBlockReason } from './ReviewStep';
 import { prefillFromPromotion, type PrefillWarning } from './promotion-prefill';
 import { useCatalog } from '../../../api/queries/catalog';
 import { useOutboundConnections } from '../../../api/queries/admin/outboundConnections';
@@ -194,7 +194,11 @@ export default function RuleEditorWizard() {
             Next
           </Button>
         ) : (
-          <Button variant="primary" onClick={onSave} disabled={create.isPending || update.isPending}>
+          <Button
+            variant="primary"
+            onClick={onSave}
+            disabled={create.isPending || update.isPending || computeSaveBlockReason(form) !== null}
+          >
             {isEdit ? 'Save changes' : 'Create rule'}
           </Button>
         )}