feat(gui): split server discovery into its own always-usable button

Previously the server settings overlay had only "Connect and sync" and
"Reset certs" — peer discovery had no button and only ran implicitly
when reopening the dialog while connected. Add a dedicated "Sincronizza"
button wired to DiscoverServersCommand, and make it work even without
an active connection by opening a short-lived connection to a candidate
server just to query its peer list, then closing it without touching
the wallet's connection state.

src/App/Views/MainView.axaml

@@ -1237,6 +1237,8 @@
                     <StackPanel Orientation="Horizontal" Spacing="8" IsVisible="{Binding IsDesktop}">
                         <Button Content="{Binding Loc[wallet.connect]}" Classes="accent"
                                 Command="{Binding ConnectAndSyncCommand}"/>
+                        <Button Content="{Binding Loc[wallet.discover]}"
+                                Command="{Binding DiscoverServersCommand}"/>
                         <Button Content="{Binding Loc[wallet.resetcert]}"
                                 Command="{Binding ResetCertificatesCommand}"/>
                     </StackPanel>
@@ -1245,6 +1247,9 @@
                         <Button Content="{Binding Loc[wallet.connect]}" Classes="accent"
                                 Command="{Binding ConnectAndSyncCommand}"
                                 HorizontalAlignment="Stretch" HorizontalContentAlignment="Center"/>
+                        <Button Content="{Binding Loc[wallet.discover]}"
+                                Command="{Binding DiscoverServersCommand}"
+                                HorizontalAlignment="Stretch" HorizontalContentAlignment="Center"/>
                         <Button Content="{Binding Loc[wallet.resetcert]}"
                                 Command="{Binding ResetCertificatesCommand}"
                                 HorizontalAlignment="Stretch" HorizontalContentAlignment="Center"/>