Initial scaffold: FocusFlow ADHD Task Manager Flutter app

BLoC/Cubit state management, ADHD-friendly theme (calming teal, no red),
GetIt DI, GoRouter navigation. Screens: task dashboard, focus mode,
task create/detail, streaks, time perception, settings, onboarding, auth.
Custom widgets: TaskCard, RewardPopup, StreakRing, GentleNudgeCard.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/core/error/failures.dart

@@ -0,0 +1,45 @@
+import 'package:equatable/equatable.dart';
+
+/// Base failure type for the application.
+///
+/// Uses a sealed class hierarchy so `switch` expressions are exhaustive.
+sealed class Failure extends Equatable {
+  final String message;
+  const Failure(this.message);
+
+  @override
+  List<Object?> get props => [message];
+}
+
+/// Returned when the backend API responds with a non-2xx status.
+class ServerFailure extends Failure {
+  final int? statusCode;
+  const ServerFailure(super.message, {this.statusCode});
+
+  @override
+  List<Object?> get props => [message, statusCode];
+}
+
+/// Returned when the device has no network connectivity.
+class NetworkFailure extends Failure {
+  const NetworkFailure([super.message = 'No internet connection. Your changes are saved locally.']);
+}
+
+/// Returned when reading / writing the local cache fails.
+class CacheFailure extends Failure {
+  const CacheFailure([super.message = 'Could not access local storage.']);
+}
+
+/// Returned for authentication problems (expired token, bad credentials, etc.).
+class AuthFailure extends Failure {
+  const AuthFailure([super.message = 'Authentication failed. Please sign in again.']);
+}
+
+/// Returned when user input does not pass validation.
+class ValidationFailure extends Failure {
+  final Map<String, String> fieldErrors;
+  const ValidationFailure(super.message, {this.fieldErrors = const {}});
+
+  @override
+  List<Object?> get props => [message, fieldErrors];
+}