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/app.slint

@@ -18,7 +18,7 @@ import { AppShell } from "shell/AppShell.slint";
 import { Toast } from "primitives/Toast.slint";
 
 // Re-export the state globals so the Rust layer can populate them.
-export { HomeState, HomeActions, DiscoverBrowseState, DiscoverBrowseActions, ForYouState, MixState, GenreFilterState, GenreFilterActions, AlbumState, ArtistState, NavState, ShellState, SessionState, SettingsState, AlbumActions, ArtistActions, ArtworkActions, NowPlayingState, QueueState, SearchState, SearchActions, NetworkSidebarState, NetworkSidebarActions, MusicianState, MusicianActions, LabelState, LabelActions, LocationViewState, LocationViewActions, FavoritesState, FavoritesActions, PlaylistPickerState, PlaylistPickerActions, PlaylistState, PlaylistActions, SidebarState, SidebarActions, SidebarFolderPopupState, CreatePlaylistState, CreatePlaylistActions, EditPlaylistState, EditPlaylistActions, CreateFolderState, CreateFolderActions, MyQbzCreateState, MyQbzCreateActions, DragState, DragActions, PlaylistManagerState, PlaylistManagerActions, OfflineManagerState, OfflineManagerActions, MyQbzState, MyQbzActions, MixtapeCardItem, MyQbzAddState, MyQbzAddActions, MyQbzAddRow, MyQbzDetailState, MyQbzDetailActions, MixtapeDetailItem, MyQbzEditState, MyQbzEditActions, MyQbzMixState, MyQbzMixActions, DiscoBuilderState, DiscoBuilderActions, DiscoGroup, DiscoCandidate, LocalLibraryState, LocalLibraryActions, LibraryFoldersState, LibFolderEditState, LibraryManageActions, LibraryScanState, LibAlbumFilterState, LocalAlbumState, LocalAlbumActions, TagEditorState, TagEditorActions, FolderEditState, FolderEditActions, ToastState, TextUtil, QconnectDevState, QconnectDevice, AppearanceState, MyQbzBrandingState, EphemeralPlayChoiceState, EphemeralPlayChoiceActions, PlexSettingsState, PlexAuthActions, PlexSectionItem, OfflineState, OfflineModeActions } from "state.slint";
+export { HomeState, HomeActions, DiscoverBrowseState, DiscoverBrowseActions, ForYouState, MixState, GenreFilterState, GenreFilterActions, AlbumState, ArtistState, NavState, ShellState, SessionState, SettingsState, AlbumActions, ArtistActions, ArtworkActions, NowPlayingState, QueueState, SearchState, SearchActions, NetworkSidebarState, NetworkSidebarActions, MusicianState, MusicianActions, LabelState, LabelActions, LocationViewState, LocationViewActions, FavoritesState, FavoritesActions, PlaylistPickerState, PlaylistPickerActions, PlaylistState, PlaylistActions, SidebarState, SidebarActions, SidebarFolderPopupState, CreatePlaylistState, CreatePlaylistActions, EditPlaylistState, EditPlaylistActions, CreateFolderState, CreateFolderActions, MyQbzCreateState, MyQbzCreateActions, DragState, DragActions, PlaylistManagerState, PlaylistManagerActions, OfflineManagerState, OfflineManagerActions, MyQbzState, MyQbzActions, MixtapeCardItem, MyQbzAddState, MyQbzAddActions, MyQbzAddRow, MyQbzDetailState, MyQbzDetailActions, MixtapeDetailItem, MyQbzEditState, MyQbzEditActions, MyQbzMixState, MyQbzMixActions, DiscoBuilderState, DiscoBuilderActions, DiscoGroup, DiscoCandidate, LocalLibraryState, LocalLibraryActions, LibraryFoldersState, LibFolderEditState, LibraryManageActions, LibraryScanState, LibAlbumFilterState, LocalAlbumState, LocalAlbumActions, TagEditorState, TagEditorActions, FolderEditState, FolderEditActions, ToastState, TextUtil, QconnectDevState, QconnectDevice, AppearanceState, MyQbzBrandingState, EphemeralPlayChoiceState, EphemeralPlayChoiceActions, PlexSettingsState, PlexAuthActions, PlexSectionItem, OfflineState, OfflineModeActions, OfflineFavoritesState, OfflineFavoritesActions } from "state.slint";
 
 // Which top-level screen is shown. The app starts on `splash` while it
 // restores a saved session, then resolves to `shell` or `login`.

crates/qbz-slint/ui/favorites/FavoritesOfflineRail.slint

@@ -0,0 +1,49 @@
+// B9 — "playable favorites" rail, shown INSIDE the offline placeholder on
+// the Favorites view only (the AppShell mounts it as the placeholder's
+// children — ADR-010: both `if`s live at the AppShell mount site). Lists
+// the favorite tracks that are still playable offline (offline-cache READY
+// copies + library qobuz_download rows, intersected with the disk-seeded
+// favorite set on the Rust side) as compact SlimCard rows; clicking a row
+// plays the whole rail from there through the offline-capable queue path.
+//
+// Collapses to nothing while the list is empty, so the placeholder then
+// reads exactly like the generic one.
+
+import { Theme } from "../foundation/semantic-colors.slint";
+import { Typography } from "../foundation/typography.slint";
+import { OfflineFavoritesState, OfflineFavoritesActions } from "../state.slint";
+import { SlimCard } from "../discover/SlimCard.slint";
+import { ListView } from "std-widgets.slint";
+
+export component FavoritesOfflineRail inherits VerticalLayout {
+    init => {
+        OfflineFavoritesActions.load();
+    }
+
+    property <int> count: OfflineFavoritesState.tracks.length;
+
+    if root.count > 0: Rectangle { height: 28px; }
+    if root.count > 0: HorizontalLayout {
+        alignment: center;
+        VerticalLayout {
+            spacing: 8px;
+            Text {
+                text: @tr("Available offline") + " (\{root.count})";
+                color: Theme.text-primary;
+                font-size: Typography.body;
+                font-weight: Typography.semibold;
+            }
+            ListView {
+                width: 480px;
+                // Up to five 60px rows visible; longer lists scroll inside.
+                height: Math.min(root.count, 5) * 60px;
+                for item in OfflineFavoritesState.tracks: SlimCard {
+                    item: item;
+                    clicked(id) => {
+                        OfflineFavoritesActions.play(id);
+                    }
+                }
+            }
+        }
+    }
+}

crates/qbz-slint/ui/offline/OfflinePlaceholder.slint

@@ -62,5 +62,10 @@ export component OfflinePlaceholder inherits Rectangle {
                 }
             }
         }
+
+        // Optional extra content under the placeholder copy. Empty on the
+        // generic mounts; the Favorites mount injects the B9 "playable
+        // favorites" rail here (see AppShell + FavoritesOfflineRail).
+        @children
     }
 }

crates/qbz-slint/ui/shell/AppShell.slint

@@ -44,6 +44,7 @@ import { LabelReleasesView } from "../label/LabelReleasesView.slint";
 import { LabelPageView } from "../label/LabelPageView.slint";
 import { ArtistsByLocationView } from "../location/ArtistsByLocationView.slint";
 import { FavoritesView } from "../favorites/FavoritesView.slint";
+import { FavoritesOfflineRail } from "../favorites/FavoritesOfflineRail.slint";
 import { MixView } from "../mix/MixView.slint";
 import { PlaylistView } from "../playlist/PlaylistView.slint";
 import { PlaylistManagerView } from "../playlist/PlaylistManagerView.slint";
@@ -183,8 +184,13 @@ export component AppShell inherits Rectangle {
 
                 // Offline: every Qobuz-only view below mounts only while
                 // online; the shared placeholder takes the pane otherwise
-                // (ADR-010 — the `if` lives here at the mount site).
-                if root.qobuz-view-blocked: OfflinePlaceholder { }
+                // (ADR-010 — the `if` lives here at the mount site). The
+                // Favorites mount alone injects the B9 "playable favorites"
+                // rail into the placeholder's children slot.
+                if root.qobuz-view-blocked && NavState.view != ContentView.favorites: OfflinePlaceholder { }
+                if root.qobuz-view-blocked && NavState.view == ContentView.favorites: OfflinePlaceholder {
+                    FavoritesOfflineRail { }
+                }
 
                 if NavState.view == ContentView.home && !OfflineState.offline: HomeView {
                     album-clicked(id) => {

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);
+}