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/WhatsNewDialog.test.jsx

@@ -50,13 +50,24 @@ describe('WhatsNewDialog', () => {
         expect(screen.queryByRole('dialog')).toBeNull();
     });
 
-    it('closes with X without marking anything seen', async () => {
+    it('marks the entries seen when closed with X', async () => {
         await showNews(NEW);
 
-        fireEvent.click(screen.getByRole('button', {name: 'Close'}));
+        fireEvent.click(screen.getByRole('button', {name: 'DIALOG.CLOSE'}));
+        await act(async () => undefined);
 
         expect(screen.queryByRole('dialog')).toBeNull();
-        expect(putCalls()).toEqual([]);
+        expect(putCalls()).toHaveLength(1);
+    });
+
+    it('marks the entries seen when closed with Esc', async () => {
+        await showNews(NEW);
+
+        fireEvent.keyDown(screen.getByRole('dialog'), {key: 'Escape'});
+        await act(async () => undefined);
+
+        expect(screen.queryByRole('dialog')).toBeNull();
+        expect(putCalls()).toHaveLength(1);
     });
 
     it.each([