Add app source and tests

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

lib/Peach/about/external/data/PeachError.dart

@@ -0,0 +1,53 @@
+import '../../internal/application/Severity.dart';
+
+/// The single error shape every failure is normalised into before it reaches
+/// the user. SSH, build and archive failures are all mapped onto this by
+/// `ParentViewModel.handleError`, so the error overlay only ever renders one
+/// kind of object.
+class PeachError {
+  final double code;
+  final String message;
+  final String helper;
+  final String title;
+  final String severity;
+
+  PeachError({
+    required this.code,
+    required this.message,
+    required this.helper,
+    required this.title,
+    required this.severity,
+  });
+
+  factory PeachError.fromJsonMap(Map<String, dynamic> json) => PeachError(
+    code: (json['code'] as num?)?.toDouble() ?? 0,
+    message: json['message'] as String? ?? '',
+    helper: json['helper'] as String? ?? '',
+    title: json['title'] as String? ?? 'Peach Error',
+    severity: json['severity'] as String? ?? Severity.message.name,
+  );
+
+  Map<String, dynamic> toJson() => {
+    'code': code,
+    'message': message,
+    'helper': helper,
+    'title': title,
+    'severity': severity,
+  };
+}
+
+/// Best-effort decode of an error payload into a [PeachError]. Anything that
+/// isn't a recognisable map is surfaced as a generic error rather than thrown,
+/// so the overlay always has something to show.
+PeachError getPeachError(dynamic data) {
+  if (data is Map<String, dynamic>) return PeachError.fromJsonMap(data);
+  return PeachError(
+    code: 9000.01,
+    message: data?.toString() ?? 'An unknown error occurred.',
+    helper:
+        'Retry the operation, and if the problem persists check the '
+        'server and your connection details.',
+    title: 'Peach Error',
+    severity: Severity.message.name,
+  );
+}