v0.6.0: delete songs from the library

Right-click a track (or a multi-selection) for "Remove from Library" or
"Remove from Library and Delete File". The second moves the file to the
desktop trash rather than unlinking it, so it stays recoverable by Ctrl+Z
in-app and by "Restore" from the file manager afterwards.

The trash is per-filesystem: music lives on a mounted volume, so the file
belongs in <topdir>/.Trash-<uid> with a topdir-relative, percent-encoded
Path. Using ~/.local/share/Trash would be a cross-device copy recording an
original path "Restore" can't reach. lintunes/trash.py implements the
freedesktop spec directly rather than adding a dependency that would need
a manual reinstall on the other machine.

Playlist cleanup is synchronous with the removal: playlist edits address
tracks by row index into track_ids while the view skips ids missing from
the library, so a dangling id would desync the two and make a later
"Remove from Playlist" hit the wrong track.

A file that can't be trashed keeps its track — better a song to delete
again than a library entry orphaned from a file still on disk. Player
gains drop_tracks() so deleting the playing track stops cleanly instead
of erroring out when the queue walk later reaches a dead id.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

TASKS.md

@@ -7,7 +7,6 @@ When a round closes, move its finished items to `tasks-done.md`.
 
 # tasks
 
-- [ ] ability to delete songs from library
 - [ ] archive the done tasks in here to another file, this is crufty....