Add createStreamFn for CORS proxy support

- createStreamFn(proxyUrl?) returns a sync streamFn that applies proxy
- Example reads proxy settings once when creating Agent
- Matches old ProviderTransport behavior

packages/web-ui/src/index.ts

@@ -112,3 +112,4 @@ export { loadAttachment } from "./utils/attachment-utils.js";
 export { clearAuthToken, getAuthToken } from "./utils/auth-token.js";
 export { formatCost, formatModelCost, formatTokenCount, formatUsage } from "./utils/format.js";
 export { i18n, setLanguage, translations } from "./utils/i18n.js";
+export { applyProxyIfNeeded, createStreamFn, isCorsError, shouldUseProxyForProvider } from "./utils/proxy-utils.js";