upload: Prevent sending messages during file uploads.

Previously, users could send messages before uploads finished,
resulting in incomplete messages with "Uploading..." text.

This commit blocks sending until uploads complete.

Fixes: #33986.

web/src/compose_validate.ts

@@ -1011,6 +1011,10 @@ export function validate(scheduling_message: boolean, show_banner = true): boole
         return false;
     }
 
+    if (upload_in_progress) {
+        return false;
+    }
+
     return true;
 }