Initial commit: CleanPlate shared Dart package

Domain models, enums, constants, validators shared between frontend and backend.

example/cleanplate_shared_example.dart

@@ -0,0 +1,13 @@
+import 'package:cleanplate_shared/cleanplate_shared.dart';
+
+void main() {
+  // Example: validate a recipe title
+  final titleError = RecipeValidator.validateTitle('My Delicious Pasta');
+  print('Title validation: ${titleError ?? "valid"}');
+
+  // Example: use diet type enum
+  print('Diet: ${DietType.vegetarian.displayName}');
+
+  // Example: API constants
+  print('Login endpoint: ${ApiConstants.authLogin}');
+}