feat: route structure, providers, layouts, and placeholder pages

app/(auth)/layout.tsx

@@ -0,0 +1,9 @@
+export default function AuthLayout({ children }: { children: React.ReactNode }) {
+  return (
+    <div className="flex min-h-screen items-center justify-center bg-muted/30">
+      <div className="w-full max-w-md rounded-lg border bg-background p-6 shadow-sm">
+        {children}
+      </div>
+    </div>
+  )
+}