fix: strip content-encoding header in proxyToGo to prevent Brotli decode error

Node fetch auto-decompresses upstream responses, but proxy_go.js was
forwarding the original content-encoding header (e.g. br/gzip) to clients.
Clients then tried to decompress already-decompressed data and failed.
Filter out content-encoding alongside content-length.

internal/js/chat-stream/proxy_go.js

@@ -53,7 +53,8 @@ async function proxyToGo(req, res, rawBody) {
 
     res.statusCode = upstream.status;
     upstream.headers.forEach((value, key) => {
-      if (key.toLowerCase() === 'content-length') {
+      const lower = key.toLowerCase();
+      if (lower === 'content-length' || lower === 'content-encoding') {
         return;
       }
       res.setHeader(key, value);