fix: preserve scroll position in cached grid views

display:none discards scrollTop in WebKitGTK, so navigating away
from album/artist/genre grids and back reset scroll to top.

Replace inline style.display toggling with a CSS class that uses
visibility:hidden + height:0 + overflow:hidden. This collapses
the element visually while keeping the DOM alive with its scroll
state intact. contain:strict on hidden views ensures zero layout
cost while collapsed.

frontend/index.css

@@ -152,3 +152,14 @@ body div.sidebar {
     height: 100%;
     contain: layout style paint;
 }
+
+/* Hidden cached views: use visibility+size collapse instead of
+   display:none so scroll containers preserve their scrollTop.
+   display:none discards scroll state in WebKitGTK. */
+.main-panel > .view-hidden {
+    visibility: hidden !important;
+    height: 0 !important;
+    overflow: hidden !important;
+    pointer-events: none !important;
+    contain: strict !important;
+}