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/pages/WelcomePage.test.jsx

@@ -0,0 +1,122 @@
+// @vitest-environment jsdom
+import {afterEach, beforeEach, describe, expect, it, vi} from 'vitest';
+import {act, cleanup, fireEvent, screen, waitFor} from '@testing-library/react';
+import {useEffect} from 'react';
+import {toast} from 'sonner';
+import WelcomePage from './WelcomePage.jsx';
+import WelcomeGate from '../components/WelcomeGate.jsx';
+import {useCurrentUserContext} from '../context/CurrentUserContext.jsx';
+import {fetchRoutes, json} from '../test/fetchRoutes.js';
+import {renderWithProviders} from '../test/renderWithProviders.jsx';
+
+vi.mock('react-i18next', () => import('../test/i18n-mock.js'));
+vi.mock('sonner', () => ({toast: {error: vi.fn()}}));
+
+const me = (overrides = {}) => ({id: 1, email: 'a@b.de', role: 'USER', steamId: null,
+    settings: {welcomeSeenAt: null}, ...overrides});
+const putCalls = () => fetch.mock.calls.filter(([, init]) => init?.method === 'PUT');
+
+const renderPage = async ({user = me(), summary = json(200, {total: 0}), seen = json(204)} = {}) => {
+    fetchRoutes({
+        'GET /api/users/me': json(200, user),
+        'GET /api/library/summary': summary,
+        'PUT /api/users/settings/welcome-seen': seen,
+    });
+    const result = renderWithProviders(<WelcomePage/>, {
+        path: '/welcome', entries: ['/welcome'], routes: {'/dashboard': <p>dashboard</p>},
+    });
+    await act(async () => undefined);
+    return result;
+};
+
+describe('WelcomePage', () => {
+    beforeEach(() => localStorage.setItem('tbp_session', '1'));
+
+    afterEach(() => {
+        cleanup();
+        localStorage.clear();
+        vi.clearAllMocks();
+    });
+
+    it('shows six feature cards, five of them with a link to their page', async () => {
+        await renderPage();
+
+        expect(screen.getAllByRole('heading', {level: 2})).toHaveLength(6);
+        expect(screen.getAllByRole('link', {name: 'WELCOME.OPEN'}).map((link) => link.getAttribute('href')))
+            .toEqual(['/library', '/library', '/stats', '/settings', '/import']);
+    });
+
+    it('checks the steps that are done', async () => {
+        await renderPage({user: me({steamId: '7656'}), summary: json(200, {total: 1})});
+
+        expect(await screen.findAllByText('WELCOME.DONE')).toHaveLength(2);
+    });
+
+    it('checks nothing for a new user without games', async () => {
+        await renderPage();
+
+        expect(screen.queryByText('WELCOME.DONE')).toBeNull();
+    });
+
+    it('renders without checks when the summary fails', async () => {
+        await renderPage({summary: json(500, {})});
+
+        expect(screen.getByText('WELCOME.TITLE')).toBeTruthy();
+        expect(screen.queryByText('WELCOME.DONE')).toBeNull();
+    });
+
+    it.each(['WELCOME.GO', 'WELCOME.SKIP'])('"%s" stamps once and opens the dashboard', async (label) => {
+        const {router} = await renderPage();
+
+        fireEvent.click(screen.getByText(label));
+
+        await waitFor(() => expect(router.state.location.pathname).toBe('/dashboard'));
+        expect(putCalls().map(([url]) => url)).toEqual([expect.stringContaining('/api/users/settings/welcome-seen')]);
+    });
+
+    it('still opens the dashboard, silently, when the stamp fails', async () => {
+        const {router} = await renderPage({seen: json(500, {})});
+
+        fireEvent.click(screen.getByText('WELCOME.GO'));
+
+        await waitFor(() => expect(router.state.location.pathname).toBe('/dashboard'));
+        expect(toast.error).not.toHaveBeenCalled();
+    });
+
+    it('staysOnTheDashboardWhenItReloadsTheUserBeforeTheStampLands', async () => {
+        let stamped = false;
+        function DashboardStub() {
+            const {reload} = useCurrentUserContext();
+            useEffect(() => { reload(); }, [reload]);
+            return <p>dashboard</p>;
+        }
+        fetchRoutes({
+            'GET /api/users/me': () => json(200, me({settings: {welcomeSeenAt: stamped ? '2026-10-01T10:00:00Z' : null}})),
+            'GET /api/library/summary': json(200, {total: 0}),
+            'PUT /api/users/settings/welcome-seen': () => new Promise((resolve) => setTimeout(() => {
+                stamped = true;
+                resolve(json(204));
+            }, 20)),
+        });
+        const {router} = renderWithProviders(<WelcomePage/>, {
+            path: '/welcome', entries: ['/welcome'],
+            routes: {'/dashboard': <WelcomeGate><DashboardStub/></WelcomeGate>},
+        });
+        await screen.findByText('WELCOME.TITLE');
+
+        fireEvent.click(screen.getByText('WELCOME.GO'));
+
+        await waitFor(() => expect(stamped).toBe(true));
+        await act(async () => new Promise((resolve) => setTimeout(resolve, 30)));
+        expect(router.state.location.pathname).toBe('/dashboard');
+    });
+
+    it('sendsTheStampWithATimeout', async () => {
+        await renderPage({seen: () => new Promise(() => undefined)});
+
+        fireEvent.click(screen.getByText('WELCOME.GO'));
+
+        await waitFor(() => expect(putCalls()).toHaveLength(1));
+        expect(putCalls()[0][1].signal).toBeInstanceOf(AbortSignal);
+    });
+});