Add custom scrollbar styles for app shell page view

Implemented Shadcn-style scrollbars for scrollable areas in the app shell. Adjusted scrollbar width and colors for both light and dark themes to enhance user experience and visual consistency.

src/routes/layout.css

@@ -162,3 +162,28 @@
 }
 
 /* Inputs inherit font size so they scale with system text size */
+
+/* Shadcn-style scrollbars for scrollable areas */
+.app-shell .page-view {
+  scrollbar-width: thin;
+  scrollbar-color: hsl(240 3.7% 15.9%) transparent;
+}
+
+.app-shell .page-view::-webkit-scrollbar {
+  width: 8px;
+}
+
+.app-shell .page-view::-webkit-scrollbar-track {
+  background: transparent;
+}
+
+.app-shell .page-view::-webkit-scrollbar-thumb {
+  background-color: hsl(240 3.7% 15.9%);
+  border-radius: 9999px;
+  border: 2px solid transparent;
+  background-clip: padding-box;
+}
+
+.dark .app-shell .page-view::-webkit-scrollbar-thumb {
+  background-color: hsl(240 3.7% 30%);
+}