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/tsconfig.json

@@ -0,0 +1,16 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "module": "ESNext",
+    "moduleResolution": "bundler",
+    "lib": ["ES2022", "DOM", "DOM.Iterable"],
+    "jsx": "react-jsx",
+    "types": ["vite/client"],
+    "strict": true,
+    "skipLibCheck": true,
+    "noEmit": true,
+    "isolatedModules": true,
+    "useDefineForClassFields": true
+  },
+  "include": ["src"]
+}