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/App/ViewModels/MainWindowViewModel.Wizard.cs

@@ -416,7 +416,7 @@ public partial class MainWindowViewModel
         }
         catch (Exception ex)
         {
-            StatusMessage = $"Errore: {ex.Message}";
+            StatusMessage = $"{Loc.Tr("msg.error")}: {ex.Message}";
         }
     }
 
@@ -473,7 +473,7 @@ public partial class MainWindowViewModel
         catch (Exception ex)
         {
             newLock.Dispose();
-            StatusMessage = $"Errore: {ex.Message}";
+            StatusMessage = $"{Loc.Tr("msg.error")}: {ex.Message}";
         }
     }