[Feature Request] Add predictive back gesture animations

[SteffoSpieler]

I'd love to see predictive back gesture animations in this app.

I found this "guide" for adding these: https://docs.flutter.dev/platform-integration/android/predictive-back
I have no experience with flutter, so I don't know if this also already handles the transitions between the screens. (Which is mainly what I'm asking here for.)

[dreautall]

It seems that the current way I'm opening basically all the sub-screens (usually a transaction, account overview or similar) is not really compatible "out of the box": OpenContainer.

It provides a nice animation from one screen to the other ("expanding" the tapped item/button), but predictive back just shows nothing. Looks like this is basically hold up by this (still draft) PR: flutter/packages#6321

As I don't really want to change the way pages are opened (and I really love the OpenContainer animation), I'll wait for that PR to be merged before this can progress further, sorry.

I'll keep the flag enabled for now so that the app theoretically would support it, as I haven't found any negative consequence for now.