Add admin dashboard shell with sidebar navigation and page scaffolds.

Replace the default Next.js landing page with a client-side AppShell layout, placeholder pages for each admin section, and UI utility dependencies.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/components/pages/WorkflowPage.tsx

@@ -0,0 +1,3 @@
+export function WorkflowPage() {
+    return <div className="text-3xl font-bold">工作流</div>;
+  }