feat(36-02): add /admin layout route and placeholder index

- admin.tsx: createFileRoute('/admin') with sidebar shell (Items, Tags disabled with Soon badge)
- admin/index.tsx: createFileRoute('/admin/') placeholder with shield icon
- useEffect guard redirects non-admin users to /

src/client/routes/admin.tsx

@@ -0,0 +1,62 @@
+import { createFileRoute, Outlet, useNavigate } from "@tanstack/react-router";
+import { useEffect } from "react";
+import { useAuth } from "../hooks/useAuth";
+import { LucideIcon } from "../lib/iconData";
+
+export const Route = createFileRoute("/admin")({
+	component: AdminLayout,
+});
+
+function AdminLayout() {
+	const navigate = useNavigate();
+	const { data: auth, isLoading } = useAuth();
+
+	useEffect(() => {
+		if (!isLoading && !auth?.user?.isAdmin) {
+			navigate({ to: "/" });
+		}
+	}, [auth, isLoading, navigate]);
+
+	// Don't render the shell until auth is confirmed
+	if (isLoading || !auth?.user?.isAdmin) return null;
+
+	return (
+		<div className="flex min-h-[calc(100vh-3.5rem)]">
+			{/* Sidebar */}
+			<aside className="w-56 border-r border-gray-100 bg-white p-4 flex flex-col gap-1 shrink-0">
+				<p className="text-xs font-semibold text-gray-400 uppercase tracking-wide mb-3">
+					Admin
+				</p>
+
+				{/* Items — disabled (phase 37) */}
+				<div
+					className="flex items-center gap-2 px-3 py-2 rounded-lg text-sm text-gray-300 cursor-not-allowed"
+					title="Coming in a future release"
+				>
+					<LucideIcon name="package" size={16} />
+					<span>Items</span>
+					<span className="ml-auto text-xs bg-gray-100 text-gray-400 px-1.5 py-0.5 rounded">
+						Soon
+					</span>
+				</div>
+
+				{/* Tags — disabled (phase 38) */}
+				<div
+					className="flex items-center gap-2 px-3 py-2 rounded-lg text-sm text-gray-300 cursor-not-allowed"
+					title="Coming in a future release"
+				>
+					<LucideIcon name="tag" size={16} />
+					<span>Tags</span>
+					<span className="ml-auto text-xs bg-gray-100 text-gray-400 px-1.5 py-0.5 rounded">
+						Soon
+					</span>
+				</div>
+			</aside>
+
+			{/* Main content */}
+			<main className="flex-1 p-6 bg-gray-50">
+				<Outlet />
+			</main>
+		</div>
+	);
+}

src/client/routes/admin/index.tsx

@@ -0,0 +1,18 @@
+import { createFileRoute } from "@tanstack/react-router";
+import { LucideIcon } from "../../lib/iconData";
+
+export const Route = createFileRoute("/admin/")({
+	component: AdminIndex,
+});
+
+function AdminIndex() {
+	return (
+		<div className="flex flex-col items-center justify-center h-64 text-center">
+			<LucideIcon name="shield" size={32} className="text-gray-300 mb-3" />
+			<p className="text-sm text-gray-500">Admin Panel</p>
+			<p className="text-xs text-gray-400 mt-1">
+				Select a section from the sidebar
+			</p>
+		</div>
+	);
+}