feat: add Cron Editor tool

Visual cron expression editor with field-by-field builder, presets
select, human-readable description, and live schedule preview showing
next occurrences. Registered in tools registry with CronIcon.

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

app/(app)/cron/page.tsx

@@ -0,0 +1,19 @@
+import { AppPage } from '@/components/layout/AppPage';
+import { CronEditor } from '@/components/cron/CronEditor';
+import { getToolByHref } from '@/lib/tools';
+import { Metadata } from 'next';
+
+const tool = getToolByHref('/cron')!;
+
+export const metadata: Metadata = {
+  title: tool.title,
+  description: tool.summary,
+};
+
+export default function CronPage() {
+  return (
+    <AppPage>
+      <CronEditor />
+    </AppPage>
+  );
+}