Completes the crate extraction (Slice 0 backend half):
- state: OfflineCacheState holder (db/fetcher/cache_dir/limit/semaphore/
library_db + dir-layout helpers + new/init_at/teardown). apply_persisted_limit
now takes Option<u64> (the caller reads it from offline_settings) instead of
borrowing src-tauri's OfflineState, so the struct is frontend-agnostic and
Slint can own one too.
- purge: purge_all_cached_files takes &Mutex<Option<LibraryDatabase>> directly
instead of src-tauri's LibraryState wrapper.
The crate is now self-contained (13 modules, no Tauri). Next: re-wire the
Tauri commands onto it with an app.emit-backed CacheEventSink and delete the
old src-tauri/src/offline_cache/ module.
cc
vicrodh
b17bbbd442