feat: Implement initial authentication, multi-environment support, and core app infrastructure.

lib/core/network/api_exceptions.dart

@@ -0,0 +1,31 @@
+class ApiException implements Exception {
+  final String message;
+  final int? statusCode;
+  final List<FieldError> fieldErrors;
+
+  const ApiException({
+    required this.message,
+    this.statusCode,
+    this.fieldErrors = const [],
+  });
+
+  @override
+  String toString() => 'ApiException: $message (status: $statusCode)';
+}
+
+class FieldError {
+  final String field;
+  final List<String> errors;
+
+  const FieldError({required this.field, required this.errors});
+
+  factory FieldError.fromJson(Map<String, dynamic> json) {
+    return FieldError(
+      field: json['field'] as String? ?? '',
+      errors: (json['errors'] as List<dynamic>?)
+              ?.map((e) => e.toString())
+              .toList() ??
+          [],
+    );
+  }
+}