docs: translate all code comments to English (language policy)

Translate every Italian /// XML doc comment, <!-- --> XAML comment, and
// inline comment to English across all source files (Core, App, tests).
Add the language policy to CLAUDE.md (conversation Italian; all code
and docs English). Update one test assertion that checked an Italian
exception message that was also translated.

src/App/ViewModels/MainWindowViewModel.Contacts.cs

@@ -13,7 +13,7 @@ public partial class MainWindowViewModel
     [ObservableProperty]
     private ContactEntry? selectedContactInList;
 
-    /// <summary>Contatto selezionato nella ComboBox del pannello Invia: riempie SendTo.</summary>
+    /// <summary>Contact selected in the Send panel's ComboBox: fills SendTo.</summary>
     [ObservableProperty]
     private ContactEntry? sendToContact;