feat: route structure, providers, layouts, and placeholder pages

app/(account)/notifications/page.tsx

@@ -0,0 +1,8 @@
+export default function NotificationsPage() {
+  return (
+    <div>
+      <h1 className="text-2xl font-bold">通知中心</h1>
+      <p className="mt-2 text-muted-foreground">订单通知、社区通知、系统通知</p>
+    </div>
+  )
+}