fix(gui): route runtime status messages through Loc instead of hardcoded Italian

Send/Donate/Sync/Wizard view models wrote status/error strings
directly in Italian ("Importo non valido.", "Errore: …", the TLS
pin-mismatch message, …), so a user running the app in English/German/
etc. still saw them in Italian despite the 6-language Loc system
covering the rest of the UI.

Added the missing Loc keys and routed every occurrence through
Loc.Tr(...). CertificatePinMismatchException (Core) no longer bakes an
Italian message into .Message; it now exposes Host/Port so the UI can
translate it, with an English fallback for any exception type the UI
doesn't specifically handle. Also drops the now-stale USERGUIDE.md
caveat about a few messages staying in Italian regardless of language.

src/Core/Net/ElectrumClient.cs

@@ -284,5 +284,9 @@ public sealed class ElectrumServerException(string error) : Exception(error);
 /// It is unlocked with an explicit reset of the certificates.
 /// </summary>
 public sealed class CertificatePinMismatchException(string host, int port) : Exception(
-    $"Il certificato TLS di {host}:{port} è cambiato rispetto a quello salvato. " +
-    "Se il server ha rinnovato il certificato, esegui il reset dei certificati SSL.");
+    $"The TLS certificate of {host}:{port} has changed from the one saved. " +
+    "If the server renewed its certificate, reset the SSL certificates.")
+{
+    public string Host { get; } = host;
+    public int Port { get; } = port;
+}