[phon-t]

Fixes #2814

Exiting a folder or album page reset the library scroll position to the top. The Albums and Folders tabs built their list/grid states with plain remember state, so the state died with the composition: for Albums when leaving Library for an album detail page, and for Folders because each folder destination inside AnimatedContent got its own fresh list state.

Changes:

• Added a pure LibraryScrollPositionStore, held in LibraryViewModel (which outlives both transitions), saving a tab's first visible item and offset on dispose and restoring it once on return.
• Albums tab (list and grid): positions saved separately; restore waits for paging to load the saved index (or reach end of pagination) instead of clamping to the first partial page.
• Folders tab: position saved per folder path plus playlist-view mode, restored once when that folder is re-entered.
• Sort changes still scroll to item 0 and are not undone by the restore.
• Added LibraryScrollPositionStoreTest covering in-range restore, empty-list null, clamping to the last item when items were removed, per-key independence, and negative-index/clear.

Testing (honest):

• The project's own Gradle test task could not be run on the machine that prepared this fix: the Gradle toolchain download for the required JetBrains JDK 21 failed from Java, and the Gradle daemon dispatch failed repeatedly, so no Gradle test result is claimed here.
• As a partial check, the exact LibraryScrollPositionStore and LibraryScrollPositionStoreTest files from this branch were compiled and run standalone with kotlinc 2.2.20 and JUnit 5.11.4: 5 tests, 5 passed, 0 failed. That is not a substitute for the project's Gradle test task, which CI should run.
• Not run on a device or emulator. Songs, Artists and Playlists tabs were left untouched, and switching to another main tab (Home/Search) and back was not the reported repro and was not verified.