Scaffold the admin PWA React shell (task 0df715b5)

Add a self-contained Vite + React + TypeScript app under admin/ that
matches the frontend/ toolchain style (vite 7, typescript 5.9, strict
TS). The app shell renders the NestQuest Admin title and the four
placeholder tabs (Today, Tasks, Schedule, History) with mobile-first
CubeCraft styling for the 390x844 reference.

- package.json scripts: dev, build, preview, test
- Vitest + jsdom + @testing-library/react smoke test proving the shell
  renders all four tab labels and switches the active tab
- commit admin/package-lock.json; ignore admin/node_modules and
  admin/dist

admin/vite.config.ts

@@ -0,0 +1,9 @@
+import { defineConfig } from "vitest/config";
+import react from "@vitejs/plugin-react";
+
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    environment: "jsdom",
+  },
+});