fix(ui): scrollbar visible in light mode

- Add light mode scrollbar thumb override with dark color
- Previously scrollbar was white (rgba(255,255,255,0.08)), invisible on light bg
- Now uses rgba(0,0,0,0.15) for light mode, visible on light backgrounds

ui/src/styles/base.css

@@ -187,6 +187,16 @@
   color-scheme: light;
 }
 
+/* Scrollbar - visible on light backgrounds */
+:root[data-theme-mode="light"] ::-webkit-scrollbar-thumb {
+  background: rgba(0, 0, 0, 0.15);
+  border-radius: var(--radius-full);
+}
+
+:root[data-theme-mode="light"] ::-webkit-scrollbar-thumb:hover {
+  background: rgba(0, 0, 0, 0.25);
+}
+
 /* Theme families override accent tokens while keeping shared surfaces/layout. */
 :root[data-theme="openknot"] {
   /*