Fix Linux audio backend preference and worker footage render crash

- On Linux, automatically prefer PipeWire/JACK/PulseAudio over ALSA
  even when a saved ALSA device name exists in config.
- Restore Footage length after deserialization so worker snapshots
  have valid stream lengths and video playback can advance.
- Guard ResolveDecoderFromInput and ProcessAudioFootage against a
  null decoder cache to prevent worker crashes on direct Footage ->
  ViewerOutput connections.
- Keep Linux signal backtrace handler in the render worker.
- Add Oak project SVG logos.

app/node/project/footage/footage.cpp

@@ -584,6 +584,11 @@ bool Footage::LoadCustom(QXmlStreamReader *reader, SerializedData *data)
 		}
 	}
 
+	// The cached lengths are not serialized. Recompute them from the stream
+	// parameters that were just loaded so that worker processes and any code
+	// that reads GetLength() before InvalidateCache() runs sees valid values.
+	VerifyLength();
+
 	return true;
 }