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.

tests/PalladiumWallet.Tests/Chain/PalladiumNetworksTests.cs

@@ -5,7 +5,7 @@ namespace PalladiumWallet.Tests.Chain;
 
 public class PalladiumNetworksTests
 {
-    // Chiave privata fissa per test deterministici (solo test, mai usarla davvero).
+    // Fixed private key for deterministic tests (test-only, never use it for real).
     private static Key TestKey => new(Convert.FromHexString(
         "0000000000000000000000000000000000000000000000000000000000000001"));