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/Views/MainView.axaml

@@ -64,11 +64,6 @@
 
                 <!-- Passo 1: scelta iniziale -->
                 <StackPanel IsVisible="{Binding IsStepStart}" Spacing="12">
-                    <StackPanel Orientation="Horizontal" Spacing="10" HorizontalAlignment="Center">
-                        <TextBlock Text="{Binding Loc[wiz.net]}" VerticalAlignment="Center"/>
-                        <ComboBox ItemsSource="{Binding Networks}"
-                                  SelectedItem="{Binding SelectedNetwork}" MinWidth="140"/>
-                    </StackPanel>
                     <Button Content="{Binding Loc[wiz.open.btn]}" FontSize="16"
                             HorizontalAlignment="Stretch" HorizontalContentAlignment="Center"
                             IsVisible="{Binding WalletFileExists}"