refactor(brewhouse): group same-page previews into PreviewGroups

Consolidate each multi-preview page into one PreviewGroup file (named <hd:Preview>
children) instead of one [Preview] class per state:
- MenuPreviews (Full Catalog / No Results)
- CartPreviews (Items / Empty)
- OrdersPreviews (History / Empty)
- ProductDetailPreviews (Today's Special / Standard)

Each child classifies by its content type (the page), so the Previews tree is unchanged.
HomePage (single preview) and the dataTemplateKey component previews (product card,
order-status badge) stay as [Preview] classes — grouping would drop their Data Templates
classification.

Verified: Debug build succeeds; the four PreviewGroup types are generated into the assembly.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Jen-Uno

studio/coffee-shop/BrewHouse/Previews/CartPageEmptyPreview.xaml

@@ -0,0 +1,25 @@
+<hd:PreviewGroup x:Class="BrewHouse.Previews.CartPreviews"
+                 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+                 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                 xmlns:local="using:BrewHouse.Presentation"
+                 xmlns:mock="using:BrewHouse.Presentation.MockData"
+                 xmlns:hd="using:Uno.UI.HotDesign"
+                 xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+                 xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+                 mc:Ignorable="d">
+
+    <!-- CartPage populated vs. empty. -->
+    <hd:Preview PreviewName="Items"
+                Visibility="Collapsed"
+                Width="390"
+                Height="844">
+        <local:CartPage DataContext="{x:Bind mock:CartPageMockData.Data}" />
+    </hd:Preview>
+
+    <hd:Preview PreviewName="Empty"
+                Visibility="Collapsed"
+                Width="390"
+                Height="844">
+        <local:CartPage DataContext="{x:Bind mock:CartPageMockData.Empty}" />
+    </hd:Preview>
+</hd:PreviewGroup>

studio/coffee-shop/BrewHouse/Previews/CartPageEmptyPreview.xaml.cs

@@ -0,0 +1,9 @@
+using Uno.UI.HotDesign;
+
+namespace BrewHouse.Previews;
+
+// A preview group holding the CartPage states (Items / Empty) as named children.
+public sealed partial class CartPreviews : PreviewGroup
+{
+    public CartPreviews() => this.InitializeComponent();
+}