feat: redesign navs

frontend/src/app/NotFoundPage.tsx

@@ -0,0 +1,21 @@
+/**
+ * 404 page for unknown routes. Shown when no route matches.
+ */
+
+import React from 'react'
+import { Link } from 'react-router-dom'
+import { Button } from '@/components/ui/button'
+
+export function NotFoundPage() {
+  return (
+    <div className="flex min-h-dvh flex-col items-center justify-center gap-6 bg-background p-8">
+      <h1 className="text-2xl font-semibold text-foreground">Page not found</h1>
+      <p className="text-sm text-muted-foreground">
+        The page you’re looking for doesn’t exist or has been moved.
+      </p>
+      <Button asChild variant="default">
+        <Link to="/recollections">Back to Recollections</Link>
+      </Button>
+    </div>
+  )
+}