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>

device_session.py

@@ -111,6 +111,10 @@ class DeviceSession:
     def is_connected(self):
         return self.sox_client is not None and not self.sox_client.is_closed
 
+    @property
+    def close_cause(self):
+        return self.sox_client.close_cause if self.sox_client else "disconnected"
+
     # ── Schema → Slot Registry ──────────────────────────────────
 
     def _load_slot_registry(self, parts):