feat: add remote products and qr code for payconic

lib/models/tab_event.dart

@@ -13,7 +13,16 @@ enum TabEventType {
 
   /// A stock count or tracking change made on the tablet. Not about a tab:
   /// it has no `tabId`.
-  stockSet('stock_set');
+  stockSet('stock_set'),
+
+  /// A product created, changed or deleted (archived) on the tablet. Not
+  /// about a tab either.
+  productCreated('product_created'),
+  productUpdated('product_updated'),
+  productArchived('product_archived');
+
+  /// The product events, which the catalog replays instead of the tabs.
+  static const products = {productCreated, productUpdated, productArchived};
 
   final String value;
 
@@ -177,6 +186,47 @@ class TabEventFactory {
     );
   }
 
+  /// A product created on this tablet: [fields] holds `name`, `category`
+  /// (null: none), `priceInCents` and `lowStockThreshold`. The id is the
+  /// tablet's, so the product can be sold before the server knows it.
+  TabEvent productCreated({
+    required String productId,
+    required Map<String, Object?> fields,
+  }) {
+    return _productEvent(TabEventType.productCreated, productId, fields);
+  }
+
+  /// Only the [fields] that changed; `imageId` is a picture uploaded first.
+  TabEvent productUpdated({
+    required String productId,
+    required Map<String, Object?> fields,
+  }) {
+    return _productEvent(TabEventType.productUpdated, productId, fields);
+  }
+
+  TabEvent productArchived({required String productId}) {
+    return _productEvent(TabEventType.productArchived, productId, null);
+  }
+
+  TabEvent _productEvent(
+    TabEventType type,
+    String productId,
+    Map<String, Object?>? fields,
+  ) {
+    final id = _uuid.v4();
+
+    return TabEvent(
+      id: id,
+      payload: {
+        'id': id,
+        'type': type.value,
+        'productId': productId,
+        'product': ?fields,
+        'occurredAt': _now().toUtc().toIso8601String(),
+      },
+    );
+  }
+
   /// The server's payment method names: `cash`, `payconiq`,
   /// `payment_pending` or `other`.
   static String paymentMethodValue(PaymentMethod method) {