feat(slint): offline content filtering and LocalLibrary offline default (D11/D12)

Offline, unavailable content is filtered out rather than shown broken:
sidebar, manager and folder popups list only local playlists and mixed
playlists with local sidecar rows (100% Qobuz hidden; empty folders
skipped); a mixed playlist opens an offline-subset detail rendering only
its local sidecar rows (plex rows under induced only, read-only, same row
machinery); MyQBZ grids and details retain only available items (qobuz =
cached album/track within grace, plex = induced only, local always) and
collections with zero available items drop from the grids.

Mixed-playlist names offline: kept from the session cache on a mid-session
flip; honest 'Playlist (N local)' fallback on a cold offline start (names
are not persisted anywhere locally - noted as a future membership-snapshot
roadmap item).

D12: an offline session entry roots navigation at LocalLibrary, and an
engine watcher auto-navigates there when the mode flips to offline while a
blocked Qobuz view is open (re-filtering open MyQBZ views in place). No
navigation on the offline-to-online edge.

cc

crates/qbz-slint/ui/state.slint

@@ -1255,6 +1255,10 @@ export global PlaylistState {
     in property <bool> is-local: false;
     // D8: never offered for upload, never reaches Qobuz/QConnect.
     in property <bool> offline-only: false;
+    // D11.a: a MIXED (Qobuz-id) playlist rendered OFFLINE from its local
+    // sidecar rows only (the Qobuz membership is not enumerable offline).
+    // Lets the AppShell mount the detail instead of the OfflinePlaceholder.
+    in property <bool> offline-subset: false;
     // Track edit mode — rows show a selection checkbox and a bulk bar
     // appears (remove selected). Only available to the owner.
     in-out property <bool> multi-select-mode: false;