revert(qbz-slint): remove hover-reveal scrollbar feature

Restore ListScrollbar, TrackRow and AlbumPageView to their pre-scrollbar
state (always-visible scrollbars). The per-row hover-reveal only revealed
the bar over track rows, not over the whole scrollable viewport, so on
mixed-content views (Artist, Label) it felt incomplete. Card sizing
(Album/Playlist/Radio at 200px) is kept.

cc

crates/qbz-slint/ui/primitives/ListScrollbar.slint

@@ -11,18 +11,9 @@ export component ListScrollbar inherits Rectangle {
     in property <length> viewport-height;
     in property <length> visible-height;
     in-out property <length> viewport-y;
-    // Hover-reveal: the owning view sets this true while the pointer is over the
-    // scrollable rows; the bar also reveals on its own gutter hover/drag. When
-    // false it fades out. DEFAULTS TRUE so views that don't wire it keep the bar
-    // always visible (no behaviour change until a view opts in).
-    in property <bool> reveal: true;
 
     // Only meaningful when the content overflows.
     property <length> max-scroll: max(0px, root.viewport-height - root.visible-height);
-
-    // Fade out unless revealed (by row hover) or the gutter itself is engaged.
-    opacity: (root.reveal || bar-ta.has-hover || bar-ta.pressed) ? 1.0 : 0.0;
-    animate opacity { duration: 150ms; }
     property <length> thumb-h:
         root.viewport-height > 0px
             ? max(48px, root.visible-height / root.viewport-height * root.height)

crates/qbz-slint/ui/primitives/TrackRow.slint

@@ -47,20 +47,12 @@ export component TrackRow inherits Rectangle {
     // Whether the row can be dragged onto sidebar playlists.
     in property <bool> draggable: true;
     callback media-action(string, string, string);
-    // Reports the row's overall hover state so a list view can reveal its
-    // scrollbar while the pointer is anywhere over the rows (the proven sidebar
-    // pattern — a background/Flickable-level TouchArea can't see hover over a
-    // row's own touch areas).
-    callback hover-changed(bool);
 
     // Explicit track number, else the 1-based list position.
     property <string> number-text:
         root.track.number != "" ? root.track.number : "\{root.index + 1}";
     property <bool> row-hovered:
         ta.has-hover || play-cell.hovered || more-ta.has-hover;
-    changed row-hovered => {
-        root.hover-changed(self.row-hovered);
-    }
 
     height: 50px;
     border-radius: 8px;