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.Android/MainApplication.cs

@@ -10,9 +10,9 @@ using PalladiumWallet.Core.Storage;
 
 namespace PalladiumWallet.Mobile;
 
-// In Avalonia 12 l'AppBuilder Android si configura nella sottoclasse Application
-// (AvaloniaAndroidApplication<TApp>), non più nell'Activity. allowBackup=false:
-// il file wallet cifrato/seed non deve finire nei backup cloud automatici.
+// In Avalonia 12 the Android AppBuilder is configured in the Application subclass
+// (AvaloniaAndroidApplication<TApp>), no longer in the Activity. allowBackup=false:
+// the encrypted wallet file/seed must not end up in automatic cloud backups.
 [Application(Label = "Palladium Wallet", AllowBackup = false,
              Icon = "@mipmap/ic_launcher", RoundIcon = "@mipmap/ic_launcher_round")]
 public class MainApplication : AvaloniaAndroidApplication<global::PalladiumWallet.App.App>
@@ -26,7 +26,7 @@ public class MainApplication : AvaloniaAndroidApplication<global::PalladiumWalle
     {
         AppPaths.OverrideDataRoot = FilesDir?.AbsolutePath;
 
-        // Registra lo scanner QR: apre ScannerActivity e ne attende il risultato.
+        // Registers the QR scanner: opens ScannerActivity and waits for its result.
         PlatformServices.ScanQrAsync = async () =>
         {
             var activity = MainActivity.Current;

src/App.Android/ScannerActivity.cs

@@ -16,8 +16,8 @@ using AndroidResult = Android.App.Result;
 namespace PalladiumWallet.Mobile;
 
 /// <summary>
-/// Activity full-screen per la scansione QR via Camera2 + ZXing.Net.
-/// Torna a MainActivity via SetResult con l'extra "qr" = testo del codice.
+/// Full-screen activity for QR scanning via Camera2 + ZXing.Net.
+/// Returns to MainActivity via SetResult with the extra "qr" = code text.
 /// </summary>
 [Activity(Theme = "@style/MyTheme.NoActionBar",
           ScreenOrientation = Android.Content.PM.ScreenOrientation.Portrait)]
@@ -162,7 +162,7 @@ internal sealed class ScannerActivity : Activity, TextureView.ISurfaceTextureLis
             bmp.GetPixels(pixels, 0, w, 0, 0, w, h);
             bmp.Recycle();
 
-            // ARGB int[] → RGB byte[] per ZXing.RGBLuminanceSource
+            // ARGB int[] → RGB byte[] for ZXing.RGBLuminanceSource
             var rgb = new byte[pixels.Length * 3];
             for (int i = 0; i < pixels.Length; i++)
             {