fix(admin): AdminShell needs "use client" (calls useMediaQuery hook)

Production error digest 4233228922: `An error occurred in the Server Components
render` on /admin/* — AdminShell was implicitly a server component but called
the useMediaQuery() client hook from its body, throwing at render time. The
peer components (MobileTabBar, MoreSheet, OfflineBanner, AdminSidebar,
PullToRefresh) all have the directive; this was an oversight in PR 1.

Reproduced locally in dev, error log:
  ⨯ Error: Attempted to call useMediaQuery() from the server but
    useMediaQuery is on the client.
    at AdminShell (src/components/admin/AdminShell.tsx:17:34)

Fix: add "use client" directive at the top of AdminShell.tsx.

src/components/admin/AdminShell.tsx

@@ -1,3 +1,5 @@
+"use client";
+
 import { ReactNode } from "react";
 import AdminSidebar from "@/components/admin/AdminSidebar";
 import MobileTabBar from "@/components/admin/MobileTabBar";