feat: alphabet list, change Layout etc

lib/models/product.dart

@@ -4,6 +4,9 @@ class Product {
   final String category;
   final int stockQuantity;
   final int lowStockThreshold;
+
+  /// Never runs out; [stockQuantity] is left untouched by sales.
+  final bool infiniteStock;
   final int priceInCents;
   final String? imagePath;
   final bool active;
@@ -14,12 +17,15 @@ class Product {
     required this.category,
     required this.stockQuantity,
     required this.lowStockThreshold,
+    this.infiniteStock = false,
     required this.priceInCents,
     this.imagePath,
     this.active = true,
   });
 
-  bool get isLowStock => stockQuantity <= lowStockThreshold;
+  bool get isOutOfStock => !infiniteStock && stockQuantity <= 0;
+
+  bool get isLowStock => !infiniteStock && stockQuantity <= lowStockThreshold;
 
   Product copyWith({
     String? id,
@@ -27,6 +33,7 @@ class Product {
     String? category,
     int? stockQuantity,
     int? lowStockThreshold,
+    bool? infiniteStock,
     int? priceInCents,
     String? imagePath,
     bool? active,
@@ -37,6 +44,7 @@ class Product {
       category: category ?? this.category,
       stockQuantity: stockQuantity ?? this.stockQuantity,
       lowStockThreshold: lowStockThreshold ?? this.lowStockThreshold,
+      infiniteStock: infiniteStock ?? this.infiniteStock,
       priceInCents: priceInCents ?? this.priceInCents,
       imagePath: imagePath ?? this.imagePath,
       active: active ?? this.active,
@@ -52,6 +60,7 @@ class Product {
           category == other.category &&
           stockQuantity == other.stockQuantity &&
           lowStockThreshold == other.lowStockThreshold &&
+          infiniteStock == other.infiniteStock &&
           priceInCents == other.priceInCents &&
           imagePath == other.imagePath &&
           active == other.active;
@@ -63,6 +72,7 @@ class Product {
     category,
     stockQuantity,
     lowStockThreshold,
+    infiniteStock,
     priceInCents,
     imagePath,
     active,

lib/models/update_failure.dart

@@ -0,0 +1,11 @@
+/// Why an over-the-air update stopped. Views translate it with
+/// `AppLocalizations.updateFailureText`.
+enum UpdateFailure {
+  download,
+  checksum,
+  permission,
+  install,
+  alreadyRunning,
+  simulated,
+  unknown,
+}