fix(studio): fix dropdown menu z-index stacking by removing transform from paper-sheet

The paper-sheet hover effect used `transform: translateY(-4px)` which
creates a new stacking context, trapping the BookMenu dropdown's z-50
inside the card. Subsequent cards in DOM order rendered above it.

Remove transform and transform-style from paper-sheet, keep the
shadow-based hover effect. Dropdown menus now render above all cards.

packages/studio/src/index.css

@@ -223,13 +223,12 @@ h3 { font-size: 1.25rem; }
   background-color: var(--card);
   border: 1px solid var(--border);
   box-shadow: var(--shadow-md);
-  transition: all 0.3s cubic-bezier(0.16, 1, 0.3, 1);
-  transform-style: preserve-3d;
+  transition: box-shadow 0.3s cubic-bezier(0.16, 1, 0.3, 1),
+              border-color 0.3s cubic-bezier(0.16, 1, 0.3, 1);
 }
 
 .paper-sheet:hover {
   box-shadow: var(--shadow-3d-hover);
-  transform: translateY(-4px) scale(1.005);
   border-color: var(--primary);
 }