fix(scoreboard): clear the two nullable warnings

Both were mine, from the scoreboard units:
- ScoreboardLayout.FitLine null-forgave tileWidths on one expression and then iterated the field
  anyway; it now binds a non-null local once and uses that throughout.
- the output resolver dereferenced _scoreboardPresenter, which is assigned in InitScoreboard — it
  is nullable by construction, so the resolver reads '?.' exactly like the other optional services.

Clean build, 0 warnings, scoreboard tests green.

ViewModels/MainViewModel.cs

@@ -596,7 +596,7 @@ public partial class MainViewModel : ViewModelBase
             WebcamSceneConfig webcam => _cameraManager.GetLatestFrame(DeviceKeyForWebcam(webcam.WebcamId), EffectKeyFor(webcam)),
             Source { IsLiveCapture: true, CaptureKey: not null } live => _screenCaptureManager.GetLatestFrame(live.CaptureKey),
             Source { Type: SourceType.ChatBox } chatBox => RenderChatBox(chatBox),
-            Source { Type: SourceType.Scoreboard } => _scoreboardPresenter.Frame,
+            Source { Type: SourceType.Scoreboard } => _scoreboardPresenter?.Frame,
             Source { Type: SourceType.AlertBox } alertBox => RenderAlertBox(alertBox),
             Source { Type: SourceType.WebSource } web => _webView2Manager?.GetLatestFrame(web.Id),
             Source { IsMediaSource: true, MediaPath: not null } media => _mediaManager.GetLatestFrame(media.MediaPath),