Slice 3/13. New qbz-offline-cache/src/purchases_service.rs (pagination glue,
get_purchase_total two-call per-type-totals primitive, byte-for-byte
filter_purchase_response). New qbz-slint/src/purchases.rs controller load shells
(get_downloaded_track_ids -> HashSet, load_purchases_metadata with TWO separate
ids-totals calls, load_purchases_by_tab with total backfill + error mapping).
Replicates Tauri's 'Failed to fetch ... purchases' wrapper so network errors map
to purchases.loadFailed 1:1 (TDD-caught). 10 unit tests green. Lazy/cache/force
gating deferred to UI slices 8/9 by design.
cc
vicrodh
bc44fe5e0c