Live mode: load sheets off the UI thread, go offline cleanly on a lost device

Opening a sheet ran its Sox requests on the UI thread, so a device that had
gone away (a closed simulator) froze the window until DASP's 30 s receive
timeout. The load now runs on a background thread and the sheet is painted
when it finishes; a load the user has clicked away from is dropped.

When the session closes, the editor goes offline with "Connection lost:
<cause>" instead of printing a traceback. A send waiting for room in the
send window now fails when the session closes rather than going out after
the CLOSE. The tree-expand error callback no longer reads the exception
after its except block has ended.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app_state.py

@@ -8,7 +8,7 @@ so later rebinding would only ever be seen by the module that did it.
 import configparser
 import os
 
-VERSION = "0.0.0.182"
+VERSION = "0.0.0.183"
 PROPERTIES_FILE = os.path.join(os.path.dirname(os.path.abspath(__file__)), "editor.properties")
 DEFAULT_GRID_SIZE = 10
 DEFAULT_ADD_TIMESTAMP = True