refactor: FaiError -> ChainError (SDK stub rename lockstep)

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/data/error_presentation.dart

@@ -32,7 +32,7 @@ import 'chain_log.dart';
 /// back to a Material default duration of 6 seconds — twice
 /// the framework default so the operator has time to click
 /// copy on an unfamiliar message.
-void showFaiErrorSnack(
+void showChainErrorSnack(
   BuildContext context,
   String source,
   Object error, {
@@ -101,7 +101,7 @@ void showFaiProcessError(
   final detail = [stderr.trim(), stdout.trim()]
       .where((s) => s.isNotEmpty)
       .join('\n\n');
-  showFaiErrorSnack(
+  showChainErrorSnack(
     context,
     source,
     detail.isEmpty ? 'process exited non-zero (no output)' : detail,
@@ -122,7 +122,7 @@ Future<void> showFaiProcessErrorDialog(
   final detail = [stderr.trim(), stdout.trim()]
       .where((s) => s.isNotEmpty)
       .join('\n\n');
-  return showFaiErrorDialog(
+  return showChainErrorDialog(
     context,
     source,
     detail.isEmpty ? 'process exited non-zero (no output)' : detail,
@@ -133,7 +133,7 @@ Future<void> showFaiProcessErrorDialog(
 /// Show [error] as a centered modal dialog. Use this when the
 /// failure blocks meaningful interaction (auth missing, hub
 /// unreachable) — a SnackBar would be too easy to dismiss.
-Future<void> showFaiErrorDialog(
+Future<void> showChainErrorDialog(
   BuildContext context,
   String source,
   Object error, {