feat(slint): TrackRow favorites + artist/album links, PlaylistView, LabelView, scrollbar

CHECKPOINT — this batch is NOT yet acceptable; substantial feedback is pending. Committed so the work is preserved for rework next session, after a max-effort Tauri code review. Areas: TrackRow favorite toggle/state + clickable artist/album (TrackItem gains is-favorite/artist-id/album-id + fav_cache, populated across all builders); PlaylistView header buttons + cover-art hover overlay; LabelView full AlbumCard + grid/list toggle; vertical scrollbar moved to the right gutter across ~12 views.

cc

crates/qbz-slint/ui/musician/MusicianPageView.slint

@@ -15,6 +15,7 @@ import {
 } from "../state.slint";
 import { QbzIcon } from "../primitives/QbzIcon.slint";
 import { LoadingSpinner } from "../primitives/LoadingSpinner.slint";
+import { ListScrollbar } from "../primitives/ListScrollbar.slint";
 import { NavButtons } from "../shell/NavButtons.slint";
 
 component AppearanceCard inherits Rectangle {
@@ -89,7 +90,7 @@ export component MusicianPageView inherits Rectangle {
 
     background: Theme.surface-main;
 
-    Flickable {
+    flick := Flickable {
         viewport-width: self.width;
         viewport-height: page.preferred-height;
 
@@ -266,4 +267,14 @@ export component MusicianPageView inherits Rectangle {
             }
         }
     }
+
+    // Shared scrollbar in the right gutter (homologated cross-view).
+    ListScrollbar {
+        x: parent.width - self.width - 4px;
+        width: 14px;
+        height: 100%;
+        viewport-height: flick.viewport-height;
+        visible-height: flick.height;
+        viewport-y <=> flick.viewport-y;
+    }
 }