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/Services/PlatformServices.cs

@@ -4,14 +4,14 @@ using System.Threading.Tasks;
 namespace PalladiumWallet.App.Services;
 
 /// <summary>
-/// Seam per servizi specifici della piattaforma (Android/desktop).
-/// Il head Android imposta i delegate in OnCreate; desktop li lascia null.
+/// Seam for platform-specific services (Android/desktop).
+/// The Android head sets the delegates in OnCreate; desktop leaves them null.
 /// </summary>
 public static class PlatformServices
 {
     /// <summary>
-    /// Apre lo scanner QR nativo e restituisce il testo raw del codice,
-    /// oppure null se l'utente annulla o lo scanner non è disponibile.
+    /// Opens the native QR scanner and returns the raw code text,
+    /// or null if the user cancels or the scanner is unavailable.
     /// </summary>
     public static Func<Task<string?>>? ScanQrAsync { get; set; }
 }