feat(welcome): add welcome page for new users after first login

Shows six feature cards and two next steps (Steam, import). The dashboard
redirects to it while Auth settings.welcomeSeenAt is null; "Let's go" and
"Skip" set the stamp. Settings links to it to reopen later.

src/components/AccountInfoCard.jsx

@@ -1,6 +1,8 @@
+import {Link} from 'react-router-dom';
 import {useTranslation} from 'react-i18next';
-import {UserRound} from 'lucide-react';
+import {Rocket, UserRound} from 'lucide-react';
 
+import {Button} from '@/components/ui/button';
 import {Card, CardContent, CardHeader, CardTitle} from '@/components/ui/card';
 import {useCurrentUserContext} from '../context/CurrentUserContext.jsx';
 import {formatDateTime} from '../utils/format.js';
@@ -38,6 +40,9 @@ export default function AccountInfoCard() {
                         </div>
                     ))}
                 </dl>
+                <Button asChild variant="link" className="mt-3 h-auto p-0">
+                    <Link to="/welcome"><Rocket/>{t('SETTINGS.OPEN_WELCOME')}</Link>
+                </Button>
             </CardContent>
         </Card>
     );

src/components/WelcomeGate.jsx

@@ -0,0 +1,11 @@
+import {Navigate} from 'react-router-dom';
+import {useCurrentUserContext} from '../context/CurrentUserContext.jsx';
+
+/**
+ * Opens the welcome page for a user who has not seen it. Only null counts: undefined is "still loading".
+ * Relies on Auth sending `welcomeSeenAt: null` explicitly (no NON_NULL Jackson config).
+ */
+export default function WelcomeGate({children}) {
+    const {user} = useCurrentUserContext();
+    return user?.settings?.welcomeSeenAt === null ? <Navigate to="/welcome" replace/> : children;
+}

src/components/WelcomeGate.test.jsx

@@ -0,0 +1,46 @@
+// @vitest-environment jsdom
+import {afterEach, beforeEach, describe, expect, it, vi} from 'vitest';
+import {cleanup, screen, waitFor} from '@testing-library/react';
+import WelcomeGate from './WelcomeGate.jsx';
+import {fetchRoutes, json} from '../test/fetchRoutes.js';
+import {renderWithProviders} from '../test/renderWithProviders.jsx';
+
+vi.mock('react-i18next', () => import('../test/i18n-mock.js'));
+
+const me = (settings) => ({id: 1, email: 'a@b.de', role: 'USER', settings});
+
+const renderGate = (settings) => {
+    fetchRoutes({'GET /api/users/me': json(200, me(settings))});
+    return renderWithProviders(<WelcomeGate><p>dashboard</p></WelcomeGate>, {
+        path: '/dashboard', entries: ['/dashboard'], routes: {'/welcome': <p>welcome</p>},
+    });
+};
+
+describe('WelcomeGate', () => {
+    beforeEach(() => localStorage.setItem('tbp_session', '1'));
+
+    afterEach(() => {
+        cleanup();
+        localStorage.clear();
+    });
+
+    it('opens the welcome page when the user has not seen it', async () => {
+        const {router} = renderGate({welcomeSeenAt: null});
+
+        await screen.findByText('welcome');
+        expect(router.state.location.pathname).toBe('/welcome');
+    });
+
+    it('shows the children once the welcome page was seen', async () => {
+        renderGate({welcomeSeenAt: '2026-10-01T10:00:00Z'});
+
+        expect(await screen.findByText('dashboard')).toBeTruthy();
+    });
+
+    it('shows the children when /me has no such field', async () => {
+        const {router} = renderGate({});
+
+        expect(await screen.findByText('dashboard')).toBeTruthy();
+        await waitFor(() => expect(router.state.location.pathname).toBe('/dashboard'));
+    });
+});