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/components/StatCard.test.jsx

@@ -0,0 +1,26 @@
+// @vitest-environment jsdom
+import {afterEach, describe, expect, it, vi} from 'vitest';
+import {cleanup, render, screen} from '@testing-library/react';
+import StatCard from './StatCard.jsx';
+
+vi.mock('react-i18next', () => import('../test/i18n-mock.js'));
+
+afterEach(() => {
+    cleanup();
+    delete document.documentElement.dataset.reduceMotion;
+});
+
+describe('StatCard', () => {
+    it('shows the final number at once when the user wants reduced motion', () => {
+        document.documentElement.dataset.reduceMotion = 'true';
+        render(<StatCard label="Games" value={1234}/>);
+
+        expect(screen.getByText('1.234')).toBeTruthy();
+    });
+
+    it('counts up from 0 otherwise', () => {
+        render(<StatCard label="Games" value={1234}/>);
+
+        expect(screen.getByText('0')).toBeTruthy();
+    });
+});