Minor ENH: bump up the opacity of labels layers to 0.6

[psobolewskiPhD]

I mentioned this here:
#8 (comment)

For the images I tested and even the Cells 3D example, the 0.3 opacity was too low for me to really see that the segmentation worked!
In this PR I bump it to 0.6 which to me looks pretty good with the new aggregation too.
(tested locally)

[FabianIsensee]

If you modify the opacity of the nnInteractive label layer this change should stick for the entire session. I think this is a more reasonable approach. Other segmentation problems require the lower opacity on order to properly see the objects border while iteratively refining the segmentation. We initially had a higher opacity and lowered it because we noticed people making errors because they could no longer see the image well enough.
What are your thoughts @Lars-Kraemer ?

[psobolewskiPhD]

That totally makes sense. It might be hard to find one universal value.
My issue was that I had a hard time telling if the segmentation worked, because the labels were so subtle.
Totally fine with y'all closing this. Thanks for the awesome tool!