Move Branta send validation toggle into Send modal and remove settings-level flag

[Copilot]

Branta validation opt-in is now controlled directly in the Send popup instead of global Settings. The validation trigger is local to each send attempt, with the checkbox defaulting to off.

• Problem alignment

  • Users wanted Branta validation enabled/disabled at send time in the popup itself, not as a persisted app setting.
  • Settings-level Branta controls were removed to avoid conflicting sources of truth.

• Send modal changes

  • Added a small CheckBox (BrantaValidationToggle) to SendFundsModal.axaml.
  • Default state is unchecked.
  • Updated ValidateBrantaAsync() in SendFundsModal.axaml.cs to run Branta validation only when the checkbox is checked.

• Settings rollback

  • Removed the Branta validation card/toggle from SettingsView.axaml.
  • Removed IsBrantaSendValidationEnabled from SettingsViewModel.cs.
  • Removed persisted Branta setting state from PrototypeSettings in ShellViewModel.cs (load/save/subscription/data contract).

• Behavioral impact

  • Branta validation is now per-send and explicit.
  • No persisted Branta validation preference remains in app settings.

private async Task<bool> ValidateBrantaAsync()
{
    if (BrantaValidationToggle.IsChecked != true)
    {
        return true;
    }

    var destination = AddressInput.Text?.Trim() ?? "";
    var validation = await BrantaValidationService.ValidateAsync(destination);
    if (validation.IsValid) return true;

    AddressError.Text = validation.ErrorMessage ?? "Address failed Branta validation";
    AddressError.IsVisible = true;
    return false;
}