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/components/PlanGrid.tsx

@@ -9,6 +9,7 @@ import {
 } from "../lib/queries/plan";
 import type { Project } from "../lib/queries/projects";
 import PlanRangeChips from "./PlanRangeChips";
+import { LoadingState } from "../ui";
 
 interface Props {
   data: GridData | undefined;
@@ -107,12 +108,7 @@ export default function PlanGrid({
   const today = useMemo(() => todayIso(), []);
   const catMap = useMemo(() => categoryMap(categories), [categories]);
 
-  if (!data)
-    return (
-      <div className="admin-loading">
-        <div className="admin-spinner" />
-      </div>
-    );
+  if (!data) return <LoadingState />;
   const days = eachDay(data.date_from, data.date_to);
   const users = data.users;
   // pulseKey?.nonce is included in the data-pulse attribute so a second