Fold the side rails' labels at once

When a panel opened or closed, the rails' labels eased their width, which
stretched and shrank every button in the rail. On the side rails the
labels now fold away and come back at once; only their fade eases.

Co-authored-by: Sameh Khamis <[email protected]>

src/ui/styles/rail-split.css

@@ -245,3 +245,10 @@
   border-radius: 14px;
   box-shadow: 0 6px 20px rgba(0, 0, 0, 0.45);
 }
+
+/* The side rails fold their labels away at once when a panel opens, and back
+   when it closes: easing the labels' width made every button visibly
+   stretch and shrink. Only their fade eases. */
+:is(.gev-panel-rail--left, .gev-panel-rail--right) .ui-rail__label {
+  transition: opacity var(--transition-fast);
+}