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/vite.config.ts

@@ -0,0 +1,11 @@
+import { defineConfig } from "vite";
+import react from "@vitejs/plugin-react";
+
+export default defineConfig({
+  plugins: [react()],
+  root: "frontend",
+  build: {
+    outDir: "../dist",
+    emptyOutDir: true,
+  },
+});