fix: proper dashboard layout — header + sidebar + content fill viewport

Use h-screen overflow-hidden on the root container for authenticated
views. Sidebar and content fill the remaining height below the header.
Remove footer from dashboard pages. Content scrolls inside its own
container, sidebar stays fixed in place.

components/dashboard/DashboardShell.tsx

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