feat(ui): donate tab in Help overlay

Help button now shows a two-tab overlay:
- Info: existing about/version content
- Donate: donate flow to the dev address (plm1qdq3…) with free amount
  input, prepare preview, and confirm-and-broadcast button

New MainWindowViewModel.Donate.cs partial: DonateAmount / DonatePreview
/ HasPendingDonate observable properties, PrepareDonateCommand (builds
the tx with hardcoded dev address via the existing TransactionFactory),
ConfirmDonateCommand (broadcasts and syncs). Donate state is reset on
overlay close. Six-language Loc keys added (help.tab.info/donate,
donate.desc/dev.address/amount/prepare/confirm).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/App/Localization/Loc.cs

@@ -60,6 +60,19 @@ public sealed class Loc
             "Portefeuille SPV pour la cryptomonnaie Palladium (PLM).",
             "Carteira SPV para a criptomoeda Palladium (PLM).",
             "SPV-Wallet für die Kryptowährung Palladium (PLM)."],
+        ["help.tab.info"]         = ["Info",                                                               "Info",                         "Info",                                 "Info",                                     "Info",                                     "Info"],
+        ["help.tab.donate"]       = ["Dona",                                                               "Donate",                       "Donar",                                "Faire un don",                             "Doar",                                     "Spenden"],
+        ["donate.desc"]           = [
+            "Se questo wallet ti è utile, considera una piccola donazione allo sviluppatore.",
+            "If you find this wallet useful, consider a small donation to the developer.",
+            "Si esta wallet te resulta útil, considera una pequeña donación al desarrollador.",
+            "Si ce portefeuille vous est utile, envisagez un petit don au développeur.",
+            "Se esta carteira é útil para você, considere uma pequena doação ao desenvolvedor.",
+            "Wenn Ihnen dieses Wallet nützlich ist, erwägen Sie eine kleine Spende an den Entwickler."],
+        ["donate.dev.address"]    = ["Indirizzo sviluppatore",                                             "Developer address",            "Dirección del desarrollador",          "Adresse du développeur",                   "Endereço do desenvolvedor",                "Entwickleradresse"],
+        ["donate.amount"]         = ["Importo donazione",                                                  "Donation amount",              "Monto de donación",                    "Montant du don",                           "Valor da doação",                          "Spendenbetrag"],
+        ["donate.prepare"]        = ["Prepara donazione",                                                  "Prepare donation",             "Preparar donación",                    "Préparer le don",                          "Preparar doação",                          "Spende vorbereiten"],
+        ["donate.confirm"]        = ["Conferma e invia",                                                   "Confirm and send",             "Confirmar y enviar",                   "Confirmer et envoyer",                     "Confirmar e enviar",                       "Bestätigen und senden"],
         ["settings.unit.short"]   = ["Unità",                                                             "Unit",                         "Unidad",                               "Unité",                                    "Unidade",                                  "Einheit"],
 
         // Wizard

src/App/ViewModels/MainWindowViewModel.Donate.cs

@@ -0,0 +1,96 @@
+using System;
+using System.Threading.Tasks;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+using NBitcoin;
+using PalladiumWallet.Core.Chain;
+using PalladiumWallet.Core.Net;
+using PalladiumWallet.Core.Wallet;
+
+namespace PalladiumWallet.App.ViewModels;
+
+public partial class MainWindowViewModel
+{
+    public const string DevAddress = "plm1qdq3gu2zvg9lyr8gxd6yln4wavc5tlp8prmvfay";
+
+    [ObservableProperty]
+    private string donateAmount = "";
+
+    [ObservableProperty]
+    private string donatePreview = "";
+
+    [ObservableProperty]
+    private bool hasPendingDonate;
+
+    private BuiltTransaction? _pendingDonate;
+
+    [RelayCommand]
+    private async Task PrepareDonate()
+    {
+        _pendingDonate = null;
+        HasPendingDonate = false;
+
+        if (_account is null || _doc?.Cache is null || _lastTransactions is null)
+        {
+            DonatePreview = "Sincronizza prima di inviare.";
+            return;
+        }
+        try
+        {
+            var destination = BitcoinAddress.Create(DevAddress, PalladiumNetworks.For(Net));
+            if (!CoinAmount.TryParseIn(DonateAmount.Trim(), _config.Unit, out var amount) || amount <= 0)
+            {
+                DonatePreview = "Importo non valido.";
+                return;
+            }
+            if (!decimal.TryParse(SendFeeRate, System.Globalization.NumberStyles.Any,
+                    System.Globalization.CultureInfo.InvariantCulture, out var feeRate) || feeRate <= 0)
+                feeRate = 1m;
+
+            _pendingDonate = new TransactionFactory(_account).Build(
+                _doc.Cache.Utxos, _lastTransactions, destination, amount, feeRate,
+                _doc.Cache.NextChangeIndex, sendAll: false);
+
+            DonatePreview = $"txid {_pendingDonate.Txid[..16]}… · " +
+                $"fee {Fmt(_pendingDonate.Fee.Satoshi)} " +
+                $"({_pendingDonate.Transaction.GetVirtualSize()} vB)" +
+                (_pendingDonate.Signed ? "" : " · watch-only");
+            HasPendingDonate = _pendingDonate.Signed;
+        }
+        catch (Exception ex)
+        {
+            _pendingDonate = null;
+            HasPendingDonate = false;
+            DonatePreview = $"Errore: {ex.Message}";
+        }
+        await Task.CompletedTask;
+    }
+
+    [RelayCommand]
+    private async Task ConfirmDonate()
+    {
+        if (_pendingDonate is null || _client is null)
+            return;
+        try
+        {
+            var txid = await _client.BroadcastAsync(_pendingDonate.ToHex());
+            DonatePreview = $"Grazie! txid: {txid}";
+            DonateAmount = "";
+            _pendingDonate = null;
+            HasPendingDonate = false;
+            await ConnectAndSync();
+        }
+        catch (Exception ex)
+        {
+            DonatePreview = $"Errore broadcast: {ex.Message}";
+        }
+    }
+
+    private void ResetDonate()
+    {
+        DonateAmount = "";
+        DonatePreview = "";
+        HasPendingDonate = false;
+        _pendingDonate = null;
+    }
+}

src/App/ViewModels/MainWindowViewModel.Settings.cs

@@ -70,5 +70,5 @@ public partial class MainWindowViewModel
     private void OpenHelp() => IsHelpOpen = true;
 
     [RelayCommand]
-    private void CloseHelp() => IsHelpOpen = false;
+    private void CloseHelp() { IsHelpOpen = false; ResetDonate(); }
 }

src/App/Views/MainView.axaml

@@ -1378,12 +1378,68 @@
                     <TextBlock Text="{Binding Loc[help.title]}"
                                FontSize="18" FontWeight="Bold"/>
 
+                    <TabControl>
+                        <!-- Tab: Info -->
+                        <TabItem Header="{Binding Loc[help.tab.info]}">
+                            <StackPanel Spacing="12" Margin="0,12,0,0">
                                 <StackPanel Spacing="4">
                                     <TextBlock Text="Palladium Wallet" FontSize="16" FontWeight="Bold"/>
-                        <TextBlock Text="{Binding WindowTitle}" FontSize="12" Foreground="{DynamicResource TextSecondaryBrush}"/>
+                                    <TextBlock Text="{Binding WindowTitle}" FontSize="12"
+                                               Foreground="{DynamicResource TextSecondaryBrush}"/>
+                                </StackPanel>
+                                <TextBlock Text="{Binding Loc[help.info]}" TextWrapping="Wrap"/>
+                            </StackPanel>
+                        </TabItem>
+
+                        <!-- Tab: Donate -->
+                        <TabItem Header="{Binding Loc[help.tab.donate]}">
+                            <StackPanel Spacing="12" Margin="0,12,0,0">
+                                <TextBlock Text="{Binding Loc[donate.desc]}" TextWrapping="Wrap"
+                                           Foreground="{DynamicResource TextSecondaryBrush}"/>
+
+                                <!-- Dev address (read-only) -->
+                                <StackPanel Spacing="4">
+                                    <TextBlock Text="{Binding Loc[donate.dev.address]}" FontSize="11"
+                                               Foreground="{DynamicResource TextSecondaryBrush}"/>
+                                    <SelectableTextBlock Text="{x:Static vm:MainWindowViewModel.DevAddress}"
+                                                         FontFamily="monospace" FontSize="11"
+                                                         TextWrapping="Wrap"/>
                                 </StackPanel>
 
-                    <TextBlock Text="{Binding Loc[help.info]}" TextWrapping="Wrap"/>
+                                <!-- Amount input -->
+                                <StackPanel Spacing="4">
+                                    <TextBlock Text="{Binding Loc[donate.amount]}" FontSize="11"
+                                               Foreground="{DynamicResource TextSecondaryBrush}"/>
+                                    <Grid ColumnDefinitions="*,Auto">
+                                        <TextBox Grid.Column="0"
+                                                 Text="{Binding DonateAmount}"
+                                                 PlaceholderText="{Binding UnitLabel}"/>
+                                        <TextBlock Grid.Column="1" Text="{Binding UnitLabel}"
+                                                   VerticalAlignment="Center" Margin="8,0,0,0"
+                                                   Foreground="{DynamicResource TextSecondaryBrush}"/>
+                                    </Grid>
+                                </StackPanel>
+
+                                <!-- Preview -->
+                                <SelectableTextBlock Text="{Binding DonatePreview}"
+                                                     IsVisible="{Binding DonatePreview,
+                                                         Converter={x:Static StringConverters.IsNotNullOrEmpty}}"
+                                                     FontFamily="monospace" FontSize="11"
+                                                     TextWrapping="Wrap"
+                                                     Foreground="{DynamicResource TextSecondaryBrush}"/>
+
+                                <!-- Buttons -->
+                                <StackPanel Orientation="Horizontal" Spacing="8" HorizontalAlignment="Right">
+                                    <Button Content="{Binding Loc[donate.confirm]}"
+                                            Classes="accent"
+                                            IsEnabled="{Binding HasPendingDonate}"
+                                            Command="{Binding ConfirmDonateCommand}"/>
+                                    <Button Content="{Binding Loc[donate.prepare]}"
+                                            Command="{Binding PrepareDonateCommand}"/>
+                                </StackPanel>
+                            </StackPanel>
+                        </TabItem>
+                    </TabControl>
 
                     <Button Content="{Binding Loc[addr.close]}"
                             HorizontalAlignment="Right"