fix: remember scroll position per folder (#104)

[joelkanyi]

Closes #104.

When navigating in/out of nested folders, the screen now restores the scroll position the user left, per folder path. Each folder remembers its own position so deep trees feel natural to navigate.

How it works

FoldersViewModel holds a Map<String, (index, offset)> keyed by folder path. The FoldersAndTracks composable runs a LaunchedEffect on currentFolder.path that:

1. Looks up the saved position for the path.
2. If found, waits until the lazy column has at least one item rendered, then scrollToItem(index, offset).
3. Then starts collecting scroll changes from lazyColumnState and saves them back through the VM.

The order matters. If save monitoring started immediately, the initial (0, 0) from the freshly-rebuilt paging source would overwrite the real saved value before we could restore it. .drop(1) on the snapshotFlow skips that first emission.

Path normalization

Folder.path carries a trailing slash for some paths (/home/cwilvx/Music/) and not for others (/home/cwilvx/Music). The VM already trims trailing slashes when matching breadcrumb entries, but the raw Folder.path on _currentFolder varies depending on entry path. Without normalization in our scroll map the same folder would key two entries, and a back-nav would miss the save. The fix calls trimEnd('/') on both save and lookup.

Tested on Pixel 7 Pro

Verified all the cases:

• Scroll deep in a folder, enter a subfolder, back. Restores.
• Multi-level: A → A/B → A/B/C, then back-back-back. Each level restores its own position.
• Switch tabs (Folders → Albums → Folders). Position survives.
• Mixed breadcrumb back-nav + tap-album-then-back. Both paths use the same map.

Diag logs during dev confirmed each restore start had the right (index, offset) and restore done fired immediately after items loaded.

[joelkanyi]

Superseded by #120, which combines scroll memory with per-path Flow caching. Closing in favor of the combined PR.