fix: pin sidebar to viewport with sticky positioning

Sidebar was scrolling with page content. Fix by adding sticky top-0
h-screen. Widen collapsed width to 68px to prevent icon clipping.

components/dashboard/DashboardShell.tsx

@@ -13,9 +13,9 @@ export default function DashboardShell({
   const { mobileOpen, closeMobile } = useSidebar()
 
   return (
-    <div className="flex flex-1 min-h-0">
+    <div className="flex flex-1">
       <Sidebar siteId={siteId} mobileOpen={mobileOpen} onMobileClose={closeMobile} />
-      <main className="flex-1 min-w-0 overflow-auto pb-8">
+      <main className="flex-1 min-w-0 pb-8">
         {children}
       </main>
     </div>