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/backend/package.json

@@ -0,0 +1,19 @@
+{
+  "name": "admin-backend",
+  "version": "0.1.0",
+  "private": true,
+  "scripts": {
+    "start": "tsx server.ts",
+    "dev": "tsx watch server.ts",
+    "typecheck": "tsc --noEmit"
+  },
+  "dependencies": {
+    "express": "^4.19.0"
+  },
+  "devDependencies": {
+    "@types/express": "^4.17.0",
+    "@types/node": "^22.0.0",
+    "tsx": "^4.7.0",
+    "typescript": "^5.4.0"
+  }
+}