Move apps to mobile directory structure

Relocated all app directories (client, design_system_viewer, staff) and their contents under the new 'apps/mobile' path. This change improves project organization and prepares for future platform-specific structuring.

apps/mobile/packages/core/lib/core.dart

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+library core;
+
+export 'src/domain/arguments/usecase_argument.dart';
+export 'src/domain/usecases/usecase.dart';

apps/mobile/packages/core/lib/src/domain/arguments/usecase_argument.dart

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+import 'package:equatable/equatable.dart';
+
+/// Abstract base class for all use case arguments.
+///
+/// Use case arguments are data transfer objects (DTOs) used to pass data
+/// into a use case. They must extend [Equatable] to ensure value equality.
+abstract class UseCaseArgument extends Equatable {
+  const UseCaseArgument();
+
+  @override
+  List<Object?> get props => [];
+}

apps/mobile/packages/core/lib/src/domain/usecases/usecase.dart

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+/// Abstract base class for all use cases in the application.
+///
+/// Use cases encapsulate application-specific business rules and orchestrate
+/// the flow of data to and from the entities. They are typically invoked
+/// from the presentation layer (e.g., BLoCs, ViewModels) and interact with
+/// repositories from the data layer.
+///
+/// [Input] represents the type of data passed into the use case.
+/// [Output] represents the type of data returned by the use case.
+abstract class UseCase<Input, Output> {
+  /// Executes the use case with the given [input].
+  ///
+  /// This method should contain the core business logic of the use case.
+  Future<Output> call(Input input);
+}
+
+/// Abstract base class for use cases that do not require any input.
+///
+/// [Output] represents the type of data returned by the use case.
+abstract class NoInputUseCase<Output> {
+  /// Executes the use case.
+  ///
+  /// This method should contain the core business logic of the use case.
+  Future<Output> call();
+}

apps/mobile/packages/core/pubspec.yaml

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+name: krow_core
+description: Core utilities and shared logic.
+version: 0.0.1
+publish_to: none
+resolution: workspace
+
+environment:
+  sdk: '>=3.10.0 <4.0.0'
+  flutter: ">=3.0.0"
+
+dependencies:
+  flutter:
+    sdk: flutter