chore(mui): swap remaining admin-spinner loaders for kit LoadingState

Replace the three remaining hand-written admin-loading/admin-spinner
markup blocks with the kit <LoadingState /> component:
- AdminApp.tsx Suspense fallback
- AppShell.tsx loading branch
- PlanGrid.tsx no-data branch (plan-* classes untouched)

After this the only remaining admin-* classes in TSX are
admin-rich-editor (RichEditor/Quill) and admin-rich-text-view
(sanitized HTML in Order/Invoice detail).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/AdminApp.tsx

@@ -8,6 +8,7 @@ import ErrorBoundary from "./components/ErrorBoundary";
 import AppShell from "./ui/AppShell";
 import AlertContainer from "./components/AlertContainer";
 import MuiProvider from "./ui/MuiProvider";
+import { LoadingState } from "./ui";
 import Login from "./pages/Login";
 import Dashboard from "./pages/Dashboard";
 import "./variables.css";
@@ -90,13 +91,7 @@ export default function AdminApp() {
           <QueryClientProvider client={queryClient}>
             <AlertContainer />
             <ErrorBoundary>
-              <Suspense
-                fallback={
-                  <div className="admin-loading">
-                    <div className="admin-spinner" />
-                  </div>
-                }
-              >
+              <Suspense fallback={<LoadingState />}>
                 <Routes>
                   <Route path="login" element={<Login />} />
                   {import.meta.env.DEV && UiKit && (