Add initial Next.js dashboard files

app/dashboard/customers/page.tsx

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+export default function CustomerPage() {
+  return <p>Customer Dashboard Page</p>;
+}

app/dashboard/invoices/page.tsx

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+export default function InvoicePage() {
+  return <p>Invoices Dashboard Page</p>;
+}

app/dashboard/layout.tsx

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+import SideNav from '@/app/ui/dashboard/sidenav';
+ 
+export default function layout({ children }: { children: React.ReactNode }) {
+  return (
+    <div className="flex h-screen flex-col md:flex-row md:overflow-hidden">
+      <div className="w-full flex-none md:w-64">
+        <SideNav />
+      </div>
+      <div className="flex-grow p-6 md:overflow-y-auto md:p-12">{children}</div>
+    </div>
+  );
+}

app/dashboard/page.tsx

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+export default function Page() {
+  return <p>  Dashboard Page</p>;
+}