Hoists Tauri's FavoritesCacheStore into qbz-app::settings::favorites_cache
(same per-user favorites_cache.db filename and schema, file shared across
frontends; 4 store tests). Slint's fav_cache now binds the store per user,
seeds the in-memory id set from disk at session activation (online,
restore, recovery and offline entry - fixing the start-offline cold-hearts
gap Tauri never closed), writes through on every favorite mutation, and
full-replaces the disk mirror on network refreshes. Queue hearts read the
shared cache instead of re-fetching from the network. All track-favorite
mutation paths are gated with a toast while offline (read-only hearts).
Part of offline-mode slice 3; main.rs wiring rides the slice-3 wiring
commit.
cc
vicrodh
727c3fb4ba