Default to full-resolution preview and fix audio-waveform freeze

- Use a divider of 1 (full resolution) for new sequences and for
  footage-derived viewer parameters instead of the auto-downscaling
  heuristic, so the viewer defaults to full-res preview.
- In AudioWaveformView, base the view timebase on the video frame rate
  when video is present; fall back to the audio sample rate only for
  audio-only sources. Using the audio sample rate as the view timebase
  created an enormous scene rect (time * sample_rate * scale), which
  froze the waveform view and stalled playback updates when showing
  the audio waveform.

app/widget/viewer/audiowaveformview.cpp

@@ -67,7 +67,11 @@ void AudioWaveformView::SetViewer(ViewerOutput *playback)
 		connect(playback_, &ViewerOutput::ConnectedWaveformChanged, viewport(),
 				static_cast<void (QWidget::*)()>(&QWidget::update));
 
-		SetTimebase(playback_->GetAudioParams().sample_rate_as_time_base());
+		SetTimebase(playback_->GetVideoParams().frame_rate_as_time_base());
+		if (timebase().isNull()) {
+			SetTimebase(
+				playback_->GetAudioParams().sample_rate_as_time_base());
+		}
 	}
 }