docs: add Cloudflare Sandbox SDK deployment guide (#57)

* docs: add Cloudflare Sandbox SDK deployment guide

- Add docs/deploy/cloudflare.mdx with full deployment guide
- Add examples/cloudflare/ with working Worker code
- Update docs navigation to include Cloudflare option
- Update deploy index page with Cloudflare card

The example shows how to run sandbox-agent inside a Cloudflare Sandbox
with exposed ports for API access.

Co-authored-by: Shelley <shelley@exe.dev>

* fix: guard server startup to avoid port conflicts

Add health check before starting sandbox-agent to prevent 'address already
in use' errors on subsequent requests. The isServerRunning() function probes
the health endpoint to determine if setup should be skipped.

Co-authored-by: Shelley <shelley@exe.dev>

* fix: default cloudflare/sandbox:0.7.0 (latest does not exist)

* 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

---------

Co-authored-by: Shelley <shelley@exe.dev>
Co-authored-by: Nathan Flurry <git@nathanflurry.com>
Co-authored-by: Nathan Flurry <developer@nathanflurry.com>

sdks/typescript/src/client.ts

@@ -66,7 +66,7 @@ export class SandboxAgent {
   private constructor(options: SandboxAgentConnectOptions) {
     this.baseUrl = options.baseUrl.replace(/\/$/, "");
     this.token = options.token;
-    this.fetcher = options.fetch ?? globalThis.fetch;
+    this.fetcher = options.fetch ?? globalThis.fetch.bind(globalThis);
     this.defaultHeaders = options.headers;
 
     if (!this.fetcher) {