feat(ui): hardcode mainnet, remove network selector

Drop the mainnet/testnet/regtest ComboBox from the wizard and fix
Net to always return NetKind.Mainnet. Core profiles and CLI --net
flag are untouched; the wallet file still records the network name.

src/App/ViewModels/MainWindowViewModel.Wizard.cs

@@ -341,7 +341,6 @@ public partial class MainWindowViewModel
         _walletLock?.Dispose();
         _walletLock = walletLock;
 
-        SelectedNetwork = doc.Network;
         _doc = doc;
         _account = account;
         _walletPath = path;

src/App/ViewModels/MainWindowViewModel.cs

@@ -101,13 +101,6 @@ public partial class MainWindowViewModel : ViewModelBase
 
     // ---- stato pannelli ----
 
-    public string[] Networks { get; } = ["mainnet", "testnet", "regtest"];
-
-    [ObservableProperty]
-    private string selectedNetwork = "mainnet";
-
-    partial void OnSelectedNetworkChanged(string value) => RefreshSetupState();
-
     [ObservableProperty]
     [NotifyPropertyChangedFor(nameof(IsSetupVisible))]
     private bool isWalletOpen;
@@ -130,7 +123,7 @@ public partial class MainWindowViewModel : ViewModelBase
 
     // ---- helpers ----
 
-    private NetKind Net => System.Enum.Parse<NetKind>(SelectedNetwork, ignoreCase: true);
+    private NetKind Net => NetKind.Mainnet;
     private ChainProfile Profile => ChainProfiles.For(Net);
     private ServerRegistry Registry => new(Profile, AppPaths.ServersPath(Net));