fix(14-01): downgrade main-panel from contain:strict to layout+style+paint

contain:strict includes size containment which caused a timing issue
where the flex-based main-panel height wasn't resolved before the
virtualizer measured its container, resulting in track-list rendering
at ~20% height on first load until something triggered a relayout.

frontend/index.css

@@ -145,7 +145,7 @@ body div.sidebar {
     padding: 0.25em;
     background-color: var(--yj-bg-surface, #212529);
     overflow: hidden;
-    contain: strict;
+    contain: layout style paint;
 }
 
 .main-panel > * {