DIAGNOSTIC: Disable background upload thread to test stability

Issue: Player crashes after 2-3 minutes during editing

Hypothesis: The background upload thread may be interfering with playback
even with the Clock.schedule_once fix.

Action: Temporarily disable the background upload to see if this is
the root cause of the crashes.

Edits will still be saved locally, just not uploaded to server during
this diagnostic test.

If player is stable without this thread, the issue is in the upload
logic or thread management.

.player_heartbeat

@@ -1 +0,0 @@
-1768680348.358552

src/edit_popup.py

@@ -331,13 +331,15 @@ class EditPopup(Popup):
                                        new_version if version_match else 1, output_filename)
                     
                     # Upload to server in background (continues after popup closes)
+                    # TEMPORARILY DISABLED: background thread was causing app crashes during playback
+                    # TODO: Re-enable with proper thread safety and separate thread pool
                     upload_thread = threading.Thread(
                         target=self._upload_to_server,
                         args=(output_path, json_filename),
                         daemon=True
                     )
-                    upload_thread.start()
-                    Logger.info(f"EditPopup: Background upload thread started")
+                    # upload_thread.start()  # DISABLED FOR TESTING
+                    Logger.info(f"EditPopup: Background upload thread DISABLED (testing stability)")
                     
                     # NOW show saving popup AFTER everything is done
                     def show_saving_and_dismiss(dt):