feat(ui): add Quit to File menu; hide Wallet item when no wallet is open

- File → Esci (separator + Quit at the bottom) calls IClassicDesktopStyleApplicationLifetime.Shutdown()
- Wallet menu item and mobile button now use IsVisible instead of IsEnabled:
  they disappear entirely on the wizard/setup screen and reappear once a
  wallet is open, avoiding a confusing greyed-out entry

src/App/Localization/Loc.cs

@@ -47,6 +47,7 @@ public sealed class Loc
         ["menu.file.new"]         = ["Nuovo / ripristina wallet…",                                        "New / restore wallet…",        "Nuevo / restaurar wallet…",            "Nouveau / restaurer le wallet…",           "Novo / restaurar carteira…",               "Neu / Wallet wiederherstellen…"],
         ["menu.file.open"]        = ["Apri wallet da file…",                                              "Open wallet from file…",       "Abrir wallet desde archivo…",          "Ouvrir le wallet depuis un fichier…",      "Abrir carteira de arquivo…",               "Wallet aus Datei öffnen…"],
         ["menu.file.close"]       = ["Chiudi wallet",                                                     "Close wallet",                 "Cerrar wallet",                        "Fermer le wallet",                         "Fechar carteira",                          "Wallet schließen"],
+        ["menu.file.quit"]        = ["Esci",                                                               "Quit",                         "Salir",                                "Quitter",                                  "Sair",                                     "Beenden"],
         ["menu.net"]              = ["_Rete",                                                             "_Network",                     "_Red",                                 "_Réseau",                                  "_Rede",                                    "_Netzwerk"],
         ["menu.net.discover"]     = ["Cerca altri server (peer)",                                         "Discover servers (peers)",     "Buscar otros servidores (peers)",      "Rechercher d'autres serveurs (pairs)",     "Procurar outros servidores (peers)",        "Weitere Server suchen (Peers)"],
         ["menu.net.resetcerts"]   = ["Reset certificati SSL",                                             "Reset SSL certificates",       "Restablecer certificados SSL",         "Réinitialiser les certificats SSL",        "Redefinir certificados SSL",               "SSL-Zertifikate zurücksetzen"],

src/App/ViewModels/MainWindowViewModel.Settings.cs

@@ -69,6 +69,12 @@ public partial class MainWindowViewModel
     [RelayCommand]
     private void OpenHelp() => IsHelpOpen = true;
 
+    [RelayCommand]
+    private static void QuitApp() =>
+        (Avalonia.Application.Current?.ApplicationLifetime
+            as Avalonia.Controls.ApplicationLifetimes.IClassicDesktopStyleApplicationLifetime)
+            ?.Shutdown();
+
     [RelayCommand]
     private void CloseHelp() { IsHelpOpen = false; ResetDonate(); }
 }

src/App/Views/MainView.axaml

@@ -36,10 +36,12 @@
                 <Separator/>
                 <MenuItem Header="{Binding Loc[menu.file.close]}" Command="{Binding CloseWalletCommand}"
                           IsEnabled="{Binding IsWalletOpen}"/>
+                <Separator/>
+                <MenuItem Header="{Binding Loc[menu.file.quit]}" Command="{Binding QuitAppCommand}"/>
             </MenuItem>
             <MenuItem Header="{Binding Loc[menu.wallet]}"
                       Command="{Binding OpenWalletInfoCommand}"
-                      IsEnabled="{Binding IsWalletOpen}"/>
+                      IsVisible="{Binding IsWalletOpen}"/>
             <MenuItem Header="{Binding Loc[menu.settings]}"
                       Command="{Binding OpenSettingsCommand}"/>
             <MenuItem Header="{Binding Loc[menu.help]}"
@@ -52,7 +54,7 @@
                     IsVisible="{Binding IsMobile}">
             <Button Content="{Binding Loc[menu.wallet]}"
                     Command="{Binding OpenWalletInfoCommand}"
-                    IsEnabled="{Binding IsWalletOpen}"/>
+                    IsVisible="{Binding IsWalletOpen}"/>
             <Button Content="{Binding Loc[menu.settings]}" Command="{Binding OpenSettingsCommand}"/>
             <Button Content="{Binding Loc[menu.help]}"     Command="{Binding OpenHelpCommand}"/>
         </StackPanel>