feat(qbz-slint): RadioCard 200px + hover-reveal scrollbar base

- RadioCard enlarged to 200x246 to match Album/Playlist cards; ForYouView +
  Spotlight row-width math updated to 200.
- Scrollbar hover-reveal: ListScrollbar gains a reveal prop (default true, no
  regression) and fades on opacity; TrackRow exposes hover-changed; AlbumPageView
  wired as the verified template (row-hover count -> reveal).

cc

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

@@ -11,9 +11,18 @@ 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)