chore: Single-source version from csproj <Version>

Add <Version>0.3.1</Version> to STPViewer.csproj and read the assembly
version at runtime for the window title, instead of hardcoding it in
XAML. Now bumping <Version> alone updates the title and the
publish-winapp output folder / exe name in one place.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/STPViewer/MainWindow.xaml

@@ -4,7 +4,7 @@
         xmlns:h="http://helix-toolkit.org/wpf"
         xmlns:conv="clr-namespace:STPViewer.Converters"
         xmlns:vm="clr-namespace:STPViewer.ViewModels"
-        Title="STPViewer V0.3.1 — STP/STEP 3D 檢視器"
+        Title="STPViewer — STP/STEP 3D 檢視器"
         Height="820" Width="1320"
         WindowStartupLocation="CenterScreen"
         AllowDrop="True" Drop="Window_Drop" DragOver="Window_DragOver">

src/STPViewer/MainWindow.xaml.cs

@@ -1,4 +1,5 @@
 using System.Linq;
+using System.Reflection;
 using System.Windows;
 using System.Windows.Input;
 using STPViewer.Services;
@@ -13,6 +14,9 @@ public partial class MainWindow : Window
     public MainWindow()
     {
         InitializeComponent();
+        // 版號單一來源 = csproj <Version>；標題執行時讀，避免寫死
+        var v = Assembly.GetExecutingAssembly().GetName().Version!;
+        Title = $"STPViewer V{v.Major}.{v.Minor}.{v.Build} — STP/STEP 3D 檢視器";
         DataContext = _vm;
         _vm.Attach(viewport);
         _vm.AttachOverlay(gizmoOverlay);

src/STPViewer/STPViewer.csproj

@@ -6,6 +6,7 @@
     <Nullable>enable</Nullable>
     <ImplicitUsings>enable</ImplicitUsings>
     <UseWPF>true</UseWPF>
+    <Version>0.3.1</Version>   <!-- 單一版號來源：標題與 publish-winapp 打包資料夾都讀這個 -->
   </PropertyGroup>
 
   <ItemGroup>