Block message send during active stream

Prevent sending messages via Enter key while a stream/response is active.
sendMessage() now returns early if isStreaming is true.

wwwroot/js/chat.js

@@ -319,6 +319,7 @@ async function _runStream(fetchPromise) {
 // ── Send Message ──
 
 async function sendMessage() {
+  if (isStreaming) return;
   const input = $('#input');
   const text = input.value.trim();
   if (!text && pendingFiles.length === 0 && pendingRefs.length === 0) return;