添加应用图标配置

- 设置应用图标为app.ico
- 移除了临时创建的favicon.ico文件

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

WinFormsApp1/WinFormsApp1.csproj

@@ -6,6 +6,7 @@
     <UseWindowsForms>true</UseWindowsForms>
     <AssemblyName>ExcelMerger</AssemblyName>
     <RootNamespace>ExcelMerger</RootNamespace>
+    <ApplicationIcon>app.ico</ApplicationIcon>
     <!-- 用于单文件发布的配置 -->
     <PublishSingleFile>false</PublishSingleFile>
     <SelfContained>false</SelfContained>