fix(app): derive an address private key only when revealed

Address rows and the address detail sheet no longer carry the WIF of
every listed address for the whole session. The key is derived after the
password gate and dropped when hidden, when the sheet closes or when the
wallet closes. See ADR 0037.

src/App/Views/MainView.axaml

@@ -1019,7 +1019,7 @@
                         <!-- Revealed: key text + hide button -->
                         <StackPanel IsVisible="{Binding $parent[UserControl].((vm:MainWindowViewModel)DataContext).IsAddressPrivKeyRevealed}"
                                     Spacing="6">
-                            <SelectableTextBlock Text="{Binding PrivKey}"
+                            <SelectableTextBlock Text="{Binding $parent[UserControl].((vm:MainWindowViewModel)DataContext).RevealedPrivKey}"
                                                  FontFamily="monospace" FontSize="12"
                                                  Foreground="{DynamicResource DangerBrush}"
                                                  TextWrapping="Wrap"/>