ErrorSnack extension Architecture

Convenience for surfacing errors to the user without leaking raw exception strings into the UI.

Always routes the error through userFriendlyMessage, which also logs the original via dev.log. Prefer this over building an ad-hoc ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text('...: $e'))), which violates the project rule against exposing e.toString() to users.