feat(slint): playlist snapshot (names+membership) and offline favorites rail

Qobuz playlist snapshot, captured opportunistically with zero new API
traffic: every online sidebar/manager list load upserts id+name+owner+
track_count for the user's playlists; opening a playlist detail online
full-replaces its membership track ids (refusing playlists never listed,
so viewed public playlists don't land). Offline consumers: synthesized
sidebar/manager entries resolve the persisted name (killing the cold-start
'Playlist (N local)' fallback when a snapshot exists), and the mixed-
playlist offline detail now renders the snapshot tracks that are playable
(snapshot intersect cached intersect grace) from the offline-cache index -
snapshot block first in snapshot order, sidecar local/plex block after.
D11 availability widens: a playlist with zero sidecar rows but cached
snapshot tracks now shows offline. 4 repo tests.

Offline favorites rail: the Favorites placeholder gains an 'Available
offline (N)' block - disk-cached favorite ids intersected with the
offline-cache READY index and LocalLibrary qobuz_download copies, compact
rows with the offline cover chain, click plays the rail as a queue through
the cache tier. Shared OfflinePlaceholder gains an @children slot; only
the Favorites mount injects the rail.

Single commit: the two features interleave in main.rs/state wiring and
ship together to keep every commit compiling.

cc

crates/qbz-slint/ui/state.slint

@@ -3192,3 +3192,23 @@ export global OfflineModeActions {
     // persistence + #279 snapshot path as the Settings toggle.
     callback set-offline(bool);
 }
+
+// B9 — offline Favorites "playable favorites" rail. While OFFLINE the
+// Favorites view is replaced by the OfflinePlaceholder; this rail (mounted
+// as the placeholder's children on the Favorites mount only) lists the
+// favorite tracks that are still playable — offline-cache READY copies and
+// library qobuz_download rows — so the view isn't a dead end.
+export global OfflineFavoritesState {
+    // The playable favorite tracks, display order = play order (the Rust
+    // side keeps a matching queue; a row click plays the rail from there).
+    in property <[SlimItem]> tracks: [];
+}
+
+export global OfflineFavoritesActions {
+    // (Re)build the rail from the local stores — fired by the rail's init
+    // on every mount of the Favorites offline placeholder.
+    callback load();
+    // Play the rail starting at the clicked track id (offline-capable
+    // queue path; the cached copies play through the offline tier).
+    callback play(string);
+}