ui(alerts): ReviewStep — render-preview pane for existing rules

Wire up the existing POST /alerts/rules/{id}/render-preview endpoint
so rule authors can preview their Mustache-templated notification
before saving. Available in edit mode only (new rules require save
first — endpoint is id-bound). Matches the njams gap: their rules
builder ships no in-builder preview and operators compensate with
trial-and-error save/retry.

Implementation notes:
- ReviewStep gains an optional `ruleId` prop; when present, a
  "Preview notification" button calls `useRenderPreview` (the
  existing TanStack mutation in api/queries/alertRules.ts) and
  renders title + message in a titled, read-only pane styled like
  a notification card.
- Errors surface as a DS Alert (variant=error) beneath the button.
- `RuleEditorWizard` passes `ruleId={id}` through — mirrors the
  existing TriggerStep / NotifyStep wiring.
- No stateless (/render-preview without id) variant exists on the
  backend, so for new rules the button is simply omitted.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -1,5 +1,7 @@
-import { useMemo } from 'react';
-import { Alert, Toggle } from '@cameleer/design-system';
+import { useMemo, useState } from 'react';
+import { Alert, Button, Toggle } from '@cameleer/design-system';
+import { useRenderPreview } from '../../../api/queries/alertRules';
+import { describeApiError } from '../../../api/errors';
 import { toRequest, type FormState } from './form-state';
 
 /**
@@ -24,12 +26,36 @@ export function computeSaveBlockReason(form: FormState): string | null {
 export function ReviewStep({
   form,
   setForm,
+  ruleId,
 }: {
   form: FormState;
   setForm?: (f: FormState) => void;
+  /**
+   * Present only in edit mode. When absent the notification-preview button is
+   * hidden, because the backend `/render-preview` endpoint is id-bound and
+   * has no stateless variant — rendering against an unsaved draft would
+   * require a new endpoint and is explicitly out of scope here.
+   */
+  ruleId?: string;
 }) {
   const req = toRequest(form);
   const saveBlockReason = useMemo(() => computeSaveBlockReason(form), [form]);
+
+  const previewMutation = useRenderPreview();
+  const [preview, setPreview] = useState<{ title: string; message: string } | null>(null);
+  const [previewError, setPreviewError] = useState<string | null>(null);
+
+  const onPreview = async () => {
+    setPreviewError(null);
+    try {
+      const res = await previewMutation.mutateAsync({ id: ruleId!, req: {} });
+      setPreview({ title: res.title ?? '', message: res.message ?? '' });
+    } catch (e) {
+      setPreview(null);
+      setPreviewError(describeApiError(e));
+    }
+  };
+
   return (
     <div style={{ display: 'grid', gap: 12, maxWidth: 720 }}>
       {saveBlockReason && (
@@ -69,6 +95,57 @@ export function ReviewStep({
           />
         </div>
       )}
+
+      {ruleId && (
+        <div style={{ display: 'grid', gap: 8 }}>
+          <div>
+            <Button
+              variant="secondary"
+              onClick={onPreview}
+              disabled={previewMutation.isPending}
+            >
+              {previewMutation.isPending ? 'Rendering\u2026' : 'Preview notification'}
+            </Button>
+          </div>
+          {previewError && (
+            <Alert variant="error" title="Preview failed">
+              {previewError}
+            </Alert>
+          )}
+          {preview && (
+            <div
+              aria-label="Rendered notification preview"
+              style={{
+                border: '1px solid var(--border)',
+                borderRadius: 8,
+                padding: 12,
+                background: 'var(--surface-raised, var(--surface))',
+                display: 'grid',
+                gap: 6,
+              }}
+            >
+              <div style={{ fontSize: 11, color: 'var(--text-muted)' }}>
+                Rendered notification preview
+              </div>
+              <div style={{ fontWeight: 600, fontSize: 14 }}>
+                {preview.title || <em style={{ color: 'var(--text-muted)' }}>(empty title)</em>}
+              </div>
+              <pre
+                style={{
+                  margin: 0,
+                  fontSize: 12,
+                  fontFamily: 'inherit',
+                  whiteSpace: 'pre-wrap',
+                  color: 'var(--text)',
+                }}
+              >
+                {preview.message || '(empty message)'}
+              </pre>
+            </div>
+          )}
+        </div>
+      )}
+
       <details>
         <summary>Raw request JSON</summary>
         <pre

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

@@ -143,7 +143,7 @@ export default function RuleEditorWizard() {
     ) : step === 'notify' ? (
       <NotifyStep form={form} setForm={setForm} ruleId={id} />
     ) : (
-      <ReviewStep form={form} setForm={setForm} />
+      <ReviewStep form={form} setForm={setForm} ruleId={id} />
     );
 
   return (