feat(library): collapsable multi-artist header on local album detail

Spec 2026-07-19-local-album-grouping-mode (B2). A compilation album (the
Saint Seiya case: 10 distinct track artists) now shows the single artist
line plus a '+N more artists' chevron that expands into the full
track-artist list, each entry clickable into the LocalLibrary Artists tab
(name-based open-artist). Distinct raw track artists in first-appearance
order, computed in apply_album_version into LocalAlbumState.all-artists.
Expansion is view-local and resets on album change; single-artist albums
never see the chevron. Track rows already showed their own artist
(verified, no change needed).

2 msgids ('+{} more artists', 'Show less') x 8 locales; slintcheck green.

km

crates/qbz-ui/ui/state.slint

@@ -1456,6 +1456,10 @@ export global LocalAlbumState {
     in property <string> quality-detail;  // "24-bit / 96 kHz"
     in property <string> cover-url;
     in property <image> cover;
+    // Distinct track artists in first-appearance order (compilations); the
+    // header shows the single artist line + a "+N more artists" chevron that
+    // expands into the clickable full list (spec §B2). Empty/1 = no chevron.
+    in property <[string]> all-artists: [];
     in property <bool> loading: false;
     in property <[TrackItem]> tracks: [];
     // Versions (distinct copies). >1 -> show the picker. Sorted best-quality