fix: hide auto-command placeholder in dashboard

When a session uses the auto command sentinel, the dashboard now leaves
the command label blank instead of showing a placeholder string.

src/webterm/local_server.py

@@ -340,7 +340,7 @@ class LocalClientConnector(SessionConnector):
 def _format_command_label(command: str) -> str:
     """Format command for display in UI, replacing sentinel with readable label."""
     if command == AUTO_COMMAND_SENTINEL:
-        return "(tmux persistent session)"
+        return ""
     return command