fix(api): prevent browser from coalescing concurrent stream requests

Add cache-busting query parameter to the stream fetch URL so each
tab gets a unique request and the browser cannot reuse an in-flight
response. Remove redundant Transfer-Encoding header that Go sets
automatically.

frontend/src/client.ts

@@ -120,7 +120,7 @@ export async function streamChatUpdates(
   chatId: string,
   onChunk: (chunk: MessageChunk) => void,
 ) {
-  const response = await fetch(`/api/chats/${chatId}/stream`);
+  const response = await fetch(`/api/chats/${chatId}/stream?_t=${Date.now()}`);
   return streamMessage(response, onChunk);
 }