fix(workspace): allow collapsing lane detail in single-pane layout, fix invisible scrollbar track

Detail toggle was hidden and info forced-visible whenever layout was
single-pane; user needed the ability to hide it too. Scrollbar track
color matched the page background (surface-0), making it invisible —
moved it to surface-2 so the track reads against any surface.

client/src/index.css

@@ -75,7 +75,7 @@
 @layer base {
   * {
     scrollbar-width: thin;
-    scrollbar-color: rgb(var(--border)) rgb(var(--surface-0));
+    scrollbar-color: rgb(var(--border)) rgb(var(--surface-2));
   }
 
   *::-webkit-scrollbar {
@@ -84,7 +84,7 @@
   }
 
   *::-webkit-scrollbar-track {
-    background: rgb(var(--surface-0));
+    background: rgb(var(--surface-2));
   }
 
   *::-webkit-scrollbar-thumb {