feat(mui): dev-only /ui-kit showcase proving the theme + data-theme bridge

Adds src/admin/pages/UiKit.tsx and registers a DEV-only <Route path="ui-kit">
sibling to <Route path="login">, outside the AdminLayout shell. The lazy import
uses the ternary `import.meta.env.DEV ? lazy(...) : null` pattern so Vite/Rolldown
evaluates the condition at build time and the UiKit chunk is entirely absent from
production bundles (confirmed: no UiKit-*.js in dist-client/assets after build:client).

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

src/admin/AdminApp.tsx

@@ -82,6 +82,7 @@ const WarehouseReports = lazy(() => import("./pages/WarehouseReports"));
 const WarehouseSuppliers = lazy(() => import("./pages/WarehouseSuppliers"));
 const WarehouseLocations = lazy(() => import("./pages/WarehouseLocations"));
 const WarehouseCategories = lazy(() => import("./pages/WarehouseCategories"));
+const UiKit = import.meta.env.DEV ? lazy(() => import("./pages/UiKit")) : null;
 
 export default function AdminApp() {
   return (
@@ -100,6 +101,9 @@ export default function AdminApp() {
               >
                 <Routes>
                   <Route path="login" element={<Login />} />
+                  {import.meta.env.DEV && UiKit && (
+                    <Route path="ui-kit" element={<UiKit />} />
+                  )}
                   <Route element={<AdminLayout />}>
                     <Route index element={<Dashboard />} />
                     <Route path="users" element={<Users />} />