feat(PROJ-17): Admin Dashboard Systemauslastung immer anzeigen

- Systemauslastungs-Sektion wird immer gerendert (nicht nur bei Erfolg)
- Fehlermeldung wenn /api/admin/system/stats nicht erreichbar ist
- Feature-Status auf In Review gesetzt

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

src/hooks/useAuth.ts

@@ -0,0 +1,46 @@
+"use client";
+
+import { useState, useEffect, useCallback } from "react";
+import { useRouter } from "next/navigation";
+import { getMe, type MeResponse } from "@/lib/api";
+
+interface AuthState {
+  user: MeResponse | null;
+  loading: boolean;
+  error: string | null;
+}
+
+export function useAuth(requireAdmin?: boolean) {
+  const router = useRouter();
+  const [state, setState] = useState<AuthState>({
+    user: null,
+    loading: true,
+    error: null,
+  });
+
+  const checkAuth = useCallback(async () => {
+    const token = localStorage.getItem("archivmail_token");
+    if (!token) {
+      router.replace("/");
+      return;
+    }
+
+    try {
+      const user = await getMe();
+      if (requireAdmin && user.role !== "admin") {
+        router.replace("/search");
+        return;
+      }
+      setState({ user, loading: false, error: null });
+    } catch {
+      localStorage.removeItem("archivmail_token");
+      router.replace("/");
+    }
+  }, [router, requireAdmin]);
+
+  useEffect(() => {
+    checkAuth();
+  }, [checkAuth]);
+
+  return state;
+}