fix(sync): allow server change at any time, even during an active sync

When IsSyncing and the user requests a different server, cancel the
running CancellationTokenSource instead of silently ignoring the click.
OperationCanceledException is caught separately (not an error), and
ConnectAndSync restarts automatically with the new server after the
cancelled task unwinds.

The "Connect" button no longer has IsEnabled="{Binding !IsSyncing}":
it is always clickable, and clicking it with the same server sets
_resyncRequested as before. CancellationToken is now passed to both
ElectrumClient.ConnectAsync and WalletSynchronizer.SyncOnceAsync.

src/App/Views/MainView.axaml

@@ -1233,14 +1233,14 @@
                     <!-- Actions — Desktop: in a row -->
                     <StackPanel Orientation="Horizontal" Spacing="8" IsVisible="{Binding IsDesktop}">
                         <Button Content="{Binding Loc[wallet.connect]}" Classes="accent"
-                                Command="{Binding ConnectAndSyncCommand}" IsEnabled="{Binding !IsSyncing}"/>
+                                Command="{Binding ConnectAndSyncCommand}"/>
                         <Button Content="{Binding Loc[wallet.resetcert]}"
                                 Command="{Binding ResetCertificatesCommand}"/>
                     </StackPanel>
                     <!-- Actions — Mobile: stacked -->
                     <StackPanel Spacing="6" IsVisible="{Binding IsMobile}">
                         <Button Content="{Binding Loc[wallet.connect]}" Classes="accent"
-                                Command="{Binding ConnectAndSyncCommand}" IsEnabled="{Binding !IsSyncing}"
+                                Command="{Binding ConnectAndSyncCommand}"
                                 HorizontalAlignment="Stretch" HorizontalContentAlignment="Center"/>
                         <Button Content="{Binding Loc[wallet.resetcert]}"
                                 Command="{Binding ResetCertificatesCommand}"