feat(cloudflare): add React frontend and improve deployment docs

- Add React + Vite frontend for Cloudflare example with sandbox-agent SDK
- Update ensureRunning to poll health endpoint instead of fixed wait
- Fix SDK fetch binding issue (globalThis.fetch.bind)
- Update docs with .dev.vars format warning and container caching tip
- Use containerFetch proxy pattern for reliable local dev

examples/cloudflare/wrangler.jsonc

@@ -4,10 +4,14 @@
   "main": "src/cloudflare.ts",
   "compatibility_date": "2025-01-01",
   "compatibility_flags": ["nodejs_compat"],
+  "assets": {
+    "directory": "./dist",
+    "binding": "ASSETS"
+  },
   "containers": [
     {
       "class_name": "Sandbox",
-      "image": "docker.io/cloudflare/sandbox:0.7.0",
+      "image": "./Dockerfile",
       "instance_type": "lite",
       "max_instances": 1
     }