[cuong-tran]

What & why

The only way to remove one downloaded song was the song menu's "Downloaded" toggle, which few people find (#2176), and there was no way to remove several at once. This adds both to Library › Downloaded:

• Swipe left on a song to remove its download. A red trash icon shows under the row, and it fires only once the row is dragged all the way, like the existing swipe right for Add to queue, which is unchanged.
• Multi-select (long press) gets a red Remove download row in its actions sheet. The Download row is hidden on this list, since everything in it is already downloaded.
• Song menu (⋯): the download row now names what a tap does: Remove download for a downloaded song (was "Downloaded") and Cancel download while it downloads (was "Downloading"). The icon still shows the state.

Removed songs leave the list at once, because the Downloaded list is read once rather than observed, and they leave the search results too while searching. There is no confirmation, matching the song menu's toggle and Remove from playlist, and one toast covers a whole batch. Removal runs off the main thread, since on Desktop each one scans the downloads folder.

Two fixes to the shared song row (SongFullWidthItems) came along: a swipe that is cancelled, or interrupted because the row starts playing or selection starts, now springs back instead of leaving the row pushed aside; and in right-to-left languages the row now follows the finger (offset {} mirrors there while drag amounts stay physical).

Needs maxrave-dev/core#43. Without it, a download removed on Android could be written back as "downloading": Media3 reports STATE_REMOVING first, which DownloadUtils read as downloading, and the removed song never left its download map. This PR does not bump core; bump it once #43 is merged.

Notes for review:

• Downloaded rows now swipe both ways (toward the end: Add to queue; toward the start: remove download).
• New strings remove_download and cancel_download, English only; translations go through Crowdin. Until then, other languages show these two in English where the menu used to show a translated "Downloaded" or "Downloading".
• On Desktop, Cancel download resets the song but does not stop the transfer yet, so the song comes back as downloaded when it finishes. That was already the case behind the old "Downloading" label, and it needs its own core fix.
• Removing a Smart download still lets the next daily pass download it again while it stays among the most played, the same as removing it from the song menu.

Linked issue

Closes #2176

Checklist

• This PR addresses an accepted issue (linked above)
• I wrote or personally reviewed every line of this change
• I tested it on Android and MacOS

Tested on both. Android: debug build on an emulator, with core#43 included, removing songs by swipe and by multi-select; afterwards the removed songs had no files left in the download cache, no Media3 download record, and downloadState 0, with none stuck at downloading. Desktop: the macOS build.

Screenshots