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/Localization/Loc.cs

@@ -169,9 +169,9 @@ public sealed class Loc
         // Wallet panel
         ["wallet.close"]          = ["Chiudi wallet",                                                     "Close wallet",                 "Cerrar wallet",                        "Fermer le wallet",                         "Fechar carteira",                          "Wallet schließen"],
         ["wallet.server"]         = ["Server:",                                                           "Server:",                      "Servidor:",                            "Serveur :",                                "Servidor:",                                "Server:"],
-        ["wallet.connect"]        = ["Connetti e sincronizza",                                            "Connect and sync",             "Conectar y sincronizar",               "Connecter et synchroniser",                "Conectar e sincronizar",                   "Verbinden und synchronisieren"],
+        ["wallet.connect"]        = ["Connetti",                                                          "Connect",                       "Conectar",                             "Connecter",                                "Conectar",                                  "Verbinden"],
         ["wallet.manual"]         = ["oppure host:porta manuale",                                         "or manual host:port",          "o host:puerto manual",                 "ou hôte:port manuel",                      "ou host:porta manual",                     "oder manuell host:port"],
-        ["wallet.discover"]       = ["Cerca altri server",                                                "Discover servers",             "Buscar otros servidores",              "Rechercher des serveurs",                  "Procurar servidores",                      "Server suchen"],
+        ["wallet.discover"]       = ["Sincronizza",                                                       "Sync servers",                  "Sincronizar",                          "Synchroniser",                             "Sincronizar",                              "Synchronisieren"],
         ["wallet.resetcert"]      = ["Reset cert.",                                                       "Reset certs",                  "Restablecer cert.",                    "Réinit. cert.",                            "Redefinir cert.",                          "Zert. zurücksetzen"],
         ["tab.receive"]           = ["Ricevi",                                                            "Receive",                      "Recibir",                              "Recevoir",                                 "Receber",                                  "Empfangen"],
         ["tab.history"]           = ["Storico",                                                           "History",                      "Historial",                            "Historique",                               "Histórico",                                "Verlauf"],

src/App/ViewModels/MainWindowViewModel.Sync.cs

@@ -345,17 +345,57 @@ public partial class MainWindowViewModel
             _ = ConnectAndSync();
     }
 
+    /// <summary>
+    /// Peer discovery. Always clickable: if the wallet is already connected, it reuses
+    /// that connection; otherwise it opens a short-lived connection to a candidate server
+    /// just to query its peer list, without touching the wallet's connection state.
+    /// </summary>
     [RelayCommand]
     private async Task DiscoverServers()
     {
-        if (_client is null || !_client.IsConnected)
+        if (_client is { IsConnected: true } connected)
         {
-            StatusMessage = Loc.Tr("conn.none") + ".";
+            await DiscoverServersUsing(connected);
+            return;
+        }
+
+        var (host, port) = ParseServer();
+        ElectrumClient? temp = null;
+        Exception? lastError = null;
+        foreach (var (h, p) in BuildServerCandidates(host, port))
+        {
+            try
+            {
+                var pins = new CertificatePinStore(AppPaths.CertificatePinsPath(Net));
+                temp = await ElectrumClient.ConnectAsync(h, p, UseSsl, pins);
+                lastError = null;
+                break;
+            }
+            catch (Exception ex)
+            {
+                lastError = ex;
+            }
+        }
+        if (temp is null)
+        {
+            StatusMessage = $"Errore nella scoperta peer: {lastError?.Message ?? Loc.Tr("conn.none")}";
             return;
         }
         try
         {
-            var added = await Registry.DiscoverAsync(_client);
+            await DiscoverServersUsing(temp);
+        }
+        finally
+        {
+            await temp.DisposeAsync();
+        }
+    }
+
+    private async Task DiscoverServersUsing(ElectrumClient client)
+    {
+        try
+        {
+            var added = await Registry.DiscoverAsync(client);
             var selected = SelectedKnownServer;
             RefreshServers();
             SelectedKnownServer = KnownServers.FirstOrDefault(s => s.Host == selected?.Host)

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"/>