Fix: Stream-close-warning via IPC statt async executeJavaScript

- Renderer meldet Streaming-Status synchron per IPC
- main.js prueft Status synchron im close-Handler
- Kein async Race mehr, Dialog erscheint zuverlaessig

web/src/plugins/streaming/StreamingTab.tsx

@@ -79,6 +79,11 @@ export default function StreamingTab({ data }: { data: any }) {
   useEffect(() => { isBroadcastingRef.current = isBroadcasting; }, [isBroadcasting]);
   useEffect(() => { viewingRef.current = viewing; }, [viewing]);
 
+  // Notify Electron about streaming status for close-warning
+  useEffect(() => {
+    (window as any).electronAPI?.setStreaming(isBroadcasting || viewing !== null);
+  }, [isBroadcasting, viewing]);
+
   // ── Elapsed time ticker ──
   useEffect(() => {
     const hasActive = streams.length > 0 || isBroadcasting;