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/design_system/test/design_system_test.dart

@@ -0,0 +1,12 @@
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:design_system/design_system.dart';
+
+void main() {
+  test('adds one to input values', () {
+    final calculator = Calculator();
+    expect(calculator.addOne(2), 3);
+    expect(calculator.addOne(-7), -6);
+    expect(calculator.addOne(0), 1);
+  });
+}