chore(sim): surface SFU subscribe errors in activity feed

Backend/public/mobile-sim.js

@@ -1071,6 +1071,8 @@ const connectSocket = () => {
         }, 6000);
       }
     } catch (e) {
+      const message = e?.message || 'Stream connect failed';
+      addActivity('SFU', message);
       Toast.show('Stream connect failed', 'error');
     }
   });