feat(wallet): surface immature balance separately from confirmed

Extract confirmation-threshold logic into UtxoSpendability (shared by
TransactionFactory and WalletSynchronizer) and use it to compute
ImmatureSats, so coinbase/under-confirmed amounts are shown separately
from spendable balance in the GUI and CLI instead of being lumped into
"confirmed".

src/App/Views/MainView.axaml

@@ -314,6 +314,9 @@
                     <TextBlock Text="{Binding UnconfirmedText}" Foreground="#FCD34D"
                                TextWrapping="Wrap"
                                IsVisible="{Binding UnconfirmedText, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
+                    <TextBlock Text="{Binding ImmatureText}" Foreground="#FCD34D"
+                               TextWrapping="Wrap"
+                               IsVisible="{Binding ImmatureText, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
                     <TextBlock Text="{Binding NetworkInfo}" Classes="on-hero" FontSize="12"
                                Margin="0,2,0,0"/>
                 </StackPanel>