Add Distinta Base import tab: parse .xls/.xml export to CSV with preview

Reads the ERP-exported "Distinta Base" file (SpreadsheetML XML or real
binary .xls/.xlsx via ExcelDataReader), extracts Componente/Descrizione,
writes a CSV and previews it in a new tab alongside the existing CAD
search. Also ignore the exported Excel/CSV files dropped in the repo root.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

CadSearchGui/CadSearchGui.csproj

@@ -10,6 +10,8 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.Data.Sqlite" Version="10.0.12" />
+    <PackageReference Include="ExcelDataReader.DataSet" Version="3.9.0" />
+    <PackageReference Include="System.Text.Encoding.CodePages" Version="9.0.0" />
   </ItemGroup>
 
 </Project>