feat(mui): adopt AppShell, remove AdminLayout/Sidebar/layout.css

Wires the MUI AppShell into the admin route and deletes the legacy chrome (AdminLayout, Sidebar, layout.css). Spinner CSS (.admin-spinner/.admin-loading) lives in base.css and is unaffected. tsc -b clean, build OK, 148 tests pass.

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

src/admin/AdminApp.tsx

@@ -5,7 +5,7 @@ import { AuthProvider } from "./context/AuthContext";
 import { AlertProvider } from "./context/AlertContext";
 import { queryClient } from "./lib/queryClient";
 import ErrorBoundary from "./components/ErrorBoundary";
-import AdminLayout from "./components/AdminLayout";
+import AppShell from "./ui/AppShell";
 import AlertContainer from "./components/AlertContainer";
 import MuiProvider from "./ui/MuiProvider";
 import Login from "./pages/Login";
@@ -14,7 +14,6 @@ import "./variables.css";
 import "./base.css";
 import "./forms.css";
 import "./buttons.css";
-import "./layout.css";
 import "./components.css";
 import "./tables.css";
 import "./datepicker.css";
@@ -104,7 +103,7 @@ export default function AdminApp() {
                   {import.meta.env.DEV && UiKit && (
                     <Route path="ui-kit" element={<UiKit />} />
                   )}
-                  <Route element={<AdminLayout />}>
+                  <Route element={<AppShell />}>
                     <Route index element={<Dashboard />} />
                     <Route path="users" element={<Users />} />
                     <Route path="attendance" element={<Attendance />} />