feat: move inventory to own view model, add signing keys

lib/viewmodels/inventory_view_model.dart

@@ -0,0 +1,61 @@
+import 'package:flutter/foundation.dart';
+
+import '../models/product.dart';
+import '../services/product_service.dart';
+
+class InventoryViewModel extends ChangeNotifier {
+  final ProductService productService;
+
+  InventoryViewModel({
+    required this.productService,
+  });
+
+  List<Product> _products = [];
+
+  List<Product> get products => _products;
+
+  Future<void> load() async {
+    _products = await productService.getProducts();
+    notifyListeners();
+  }
+
+  Future<void> decreaseStock(
+      String productId,
+      int amount,
+      ) async {
+    await productService.decreaseStock(productId, amount);
+
+    final index = _products.indexWhere(
+          (product) => product.id == productId,
+    );
+
+    if (index != -1) {
+      _products[index] = _products[index].copyWith(
+        stockQuantity:
+        _products[index].stockQuantity - amount,
+      );
+    }
+
+    notifyListeners();
+  }
+
+  Future<void> increaseStock(
+      String productId,
+      int amount,
+      ) async {
+    await productService.increaseStock(productId, amount);
+
+    final index = _products.indexWhere(
+          (product) => product.id == productId,
+    );
+
+    if (index != -1) {
+      _products[index] = _products[index].copyWith(
+        stockQuantity:
+        _products[index].stockQuantity + amount,
+      );
+    }
+
+    notifyListeners();
+  }
+}