Add the admin panel scaffold

A self-contained template: an Express backend with login, JWT, rate limiting
and SPA serving, and a React 18 + Vite shell with the same tab registry and
warm Material 3 design system the panel uses. No domain logic — routes and tabs
are meant to be hung off it.

node_modules and the frontend build stay out through the existing ignores.

admin/frontend/src/vite-env.d.ts

@@ -0,0 +1,4 @@
+// Copyright (c) 2026 Ivan Vasilev
+// This source code is licensed under the MIT license found in the
+// LICENSE file in the root directory of this source tree.
+/// <reference types="vite/client" />