Inlines the bodies of `check_album_fully_cached` and
`check_albums_fully_cached_batch` from
crate::offline_cache::commands::* directly into
commands_v2/legacy_compat.rs, mapping the legacy String error type to
RuntimeError::Internal at the v2 boundary.
Note: v2_cache_notification_artwork is NOT a delegate — it lives in
commands_v2/helpers.rs and is a true helper used by other v2 commands.
The investigation list flagged it as a possible delegate, but
verification showed it owns its implementation.
Deletes the legacy #[tauri::command] definitions in
offline_cache/commands.rs. Other functions in the offline_cache::commands
module remain (still called from commands_v2/library.rs delegates,
which are step 8 scope).
No frontend invoke changes required.
Step 6 of the legacy cleanup plan, group D (offline cache).
cc
vicrodh
afa8298745