Inlines bodies of library_fetch_missing_artwork, library_get_thumbnail,
and library_get_thumbnails_cache_size from library/commands.rs directly
into the v2 wrappers in commands_v2/library.rs.
Deletes the corresponding legacy #[tauri::command] definitions and the
unused legacy library_clear_thumbnails command. Removes their re-exports
from library/mod.rs.
Step 8 of legacy cleanup, group D.
cc
vicrodh
44ad3392ef