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/ui/AppShell.tsx

@@ -9,6 +9,7 @@ import { useAuth } from "../context/AuthContext";
 import ThemeToggle from "./ThemeToggle";
 import { setLogoutAlert } from "../utils/api";
 import SidebarNav from "./SidebarNav";
+import LoadingState from "./LoadingState";
 import ShortcutsHelp from "../components/ShortcutsHelp";
 
 const DRAWER_WIDTH = 248;
@@ -38,7 +39,7 @@ export default function AppShell() {
             bgcolor: "background.default",
           }}
         >
-          <div className="admin-spinner" />
+          <LoadingState />
         </Box>
       </ScopedCssBaseline>
     );