example-host: fix sandboxing setup (#21)

* example-host: prevent accidental non-iframing of sandbox (or iframing in prod)

* fix sandbox isolation + "test in prod"

* Update package.json

* Update sandbox.ts

examples/simple-host/package.json

@@ -5,7 +5,7 @@
   "type": "module",
   "scripts": {
     "start": "NODE_ENV=development npm run build && concurrently 'npm run start:server'",
-    "start:server": "bun server.ts",
+    "start:server": "bun serve.ts",
     "build": "concurrently 'INPUT=example-host-vanilla.html vite build' 'INPUT=example-host-react.html vite build' 'INPUT=sandbox.html vite build'"
   },
   "dependencies": {