fix(ux): apply UX quick wins (#54)

Get started links to registration and says an admin approves accounts.
Back navigation restores the library scroll; search no longer fills history.
Steam and PSN unlink ask first. Any close marks news as seen.
Confetti plays only when the level rises on the page.
Count-ups respect reduced motion. Html lang follows the UI language.
Add a 404 page, translated role and dialog close label.

src/pages/NotFoundPage.jsx

@@ -0,0 +1,26 @@
+import {Link} from 'react-router-dom';
+import {useTranslation} from 'react-i18next';
+
+import {Button} from '@/components/ui/button';
+import {Card, CardContent, CardHeader, CardTitle} from '@/components/ui/card';
+import useSession from '../hooks/useSession.js';
+
+export default function NotFoundPage() {
+    const {t} = useTranslation();
+    const {active} = useSession();
+
+    return (
+        <div className="w-full max-w-3xl mx-auto p-4 md:p-8">
+            <Card>
+                <CardHeader>
+                    <CardTitle className="text-3xl font-bold tracking-tight">{t('NOT_FOUND.TITLE')}</CardTitle>
+                </CardHeader>
+                <CardContent>
+                    <Button asChild>
+                        {active ? <Link to="/dashboard">{t('NAV.TO_DASHBOARD')}</Link> : <Link to="/">{t('PROFILE.TO_START')}</Link>}
+                    </Button>
+                </CardContent>
+            </Card>
+        </div>
+    );
+}