fix: Make transactions atomic / AutoLogout

lib/viewmodels/bar_screen_view_model.dart

@@ -95,16 +95,9 @@ class BarScreenViewModel extends ChangeNotifier {
 
     if (tab == null) return;
 
-    if (product.stockQuantity <= 0) {
-      throw const InsufficientStockException();
-    }
-
     try {
-      await barTabService.addProductToTab(
-        tabId: tab.id,
-        product: product,
-        stockAdjustment: () => inventory.decreaseStock(product.id, 1),
-      );
+      await barTabService.addProductToTab(tabId: tab.id, product: product);
+      inventory.applyStockDelta(product.id, -1);
 
       await _reloadTabs();
     } catch (e, stack) {
@@ -118,35 +111,15 @@ class BarScreenViewModel extends ChangeNotifier {
     }
   }
 
-  Future<void> changeItemQuantity(TabItem item, int quantity) async {
-    final difference = quantity - item.quantity;
-
-    if (difference > 0) {
-      Product? product;
-      for (final candidate in inventory.products) {
-        if (candidate.id == item.productId) {
-          product = candidate;
-          break;
-        }
-      }
-
-      if (product != null && product.stockQuantity < difference) {
-        throw const InsufficientStockException();
-      }
-    }
+  Future<void> changeItemQuantity(TabItem item, int delta) async {
+    if (delta == 0) return;
 
     try {
-      await barTabService.updateTabItemQuantity(
+      final actualDelta = await barTabService.adjustTabItemQuantity(
         tabItemId: item.id,
-        quantity: quantity,
-        stockAdjustment: () async {
-          if (difference > 0) {
-            await inventory.decreaseStock(item.productId, difference);
-          } else if (difference < 0) {
-            await inventory.increaseStock(item.productId, -difference);
-          }
-        },
+        delta: delta,
       );
+      inventory.applyStockDelta(item.productId, -actualDelta);
 
       await _reloadTabs();
     } catch (e, stack) {
@@ -160,7 +133,9 @@ class BarScreenViewModel extends ChangeNotifier {
     }
   }
 
-  Future<void> closeSelectedTab({PaymentMethod paymentMethod = PaymentMethod.cash}) async {
+  Future<void> closeSelectedTab({
+    PaymentMethod paymentMethod = PaymentMethod.cash,
+  }) async {
     final tab = selectedTab;
 
     if (tab == null) return;
@@ -177,7 +152,10 @@ class BarScreenViewModel extends ChangeNotifier {
     }
   }
 
-  Future<void> closeTab(String tabId, {PaymentMethod paymentMethod = PaymentMethod.cash}) async {
+  Future<void> closeTab(
+    String tabId, {
+    PaymentMethod paymentMethod = PaymentMethod.cash,
+  }) async {
     try {
       await barTabService.closeTab(tabId, paymentMethod: paymentMethod);
       await _reloadTabs();

lib/viewmodels/inventory_view_model.dart

@@ -65,4 +65,16 @@ class InventoryViewModel extends ChangeNotifier {
       rethrow;
     }
   }
+
+  void applyStockDelta(String productId, int delta) {
+    if (delta == 0) return;
+
+    final index = _products.indexWhere((product) => product.id == productId);
+    if (index == -1) return;
+
+    _products[index] = _products[index].copyWith(
+      stockQuantity: _products[index].stockQuantity + delta,
+    );
+    notifyListeners();
+  }
 }