fix: preserve scroll position when returning to library tabs

[stamchry]

Overview

This fixes an annoying UX issue where navigating away from a library tab (like clicking on an album in the Albums tab) and then pressing back would jump you all the way back to the top of the list.

What changed

I moved the scroll states (LazyListState and LazyGridState) out of the individual tabs (Albums, Artists, Songs) and hoisted them up to the parent LibraryScreen.

Previously, because the scroll states lived inside the tab composables themselves, they were getting wiped out whenever the Navigation system or the HorizontalPager had to recreate the view upon returning. By holding the states at the LibraryScreen level, the screen's saveable state registry can properly hold onto them while we navigate back and forth.

How to test

1. Open the app and go to the Albums tab.
2. Scroll down a bit and click on any album to open its details.
3. Press back. You should be exactly where you left off instead of at the top.