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/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(); }
 }