refactor: move scrollbar styles to global utilities

Apply thin subtle scrollbars globally via utilities.css instead of
only on .sidebar-content. Uses scrollbar-width/scrollbar-color for
Firefox and ::-webkit-scrollbar for Chrome/Safari. Removes the
now-redundant sidebar-specific scrollbar rules.

src/ui/sidebar.css

@@ -150,20 +150,6 @@
   min-width: 0;
 }
 
-/* Scrollbar styling */
-.sidebar-content::-webkit-scrollbar {
-  width: 4px;
-}
-
-.sidebar-content::-webkit-scrollbar-track {
-  background: transparent;
-}
-
-.sidebar-content::-webkit-scrollbar-thumb {
-  background: var(--bg-2);
-  border-radius: 9999px;
-}
-
 /* ── Sidebar Group ─────────────────────────────────────────────── */
 
 .sidebar-group {

src/ui/utilities.css

@@ -62,3 +62,29 @@
   white-space: nowrap;
   border: 0;
 }
+
+/* ── Scrollbars ────────────────────────────────────────────────── */
+/* Thin, subtle scrollbars everywhere.                             */
+
+* {
+  scrollbar-width: thin;
+  scrollbar-color: var(--bg-2) transparent;
+}
+
+::-webkit-scrollbar {
+  width: 4px;
+  height: 4px;
+}
+
+::-webkit-scrollbar-track {
+  background: transparent;
+}
+
+::-webkit-scrollbar-thumb {
+  background: var(--bg-2);
+  border-radius: 9999px;
+}
+
+::-webkit-scrollbar-thumb:hover {
+  background: var(--fg-2);
+}