CSS class migrations: admin views and boot overlay fade-out

- Replace style.display with .is-hidden classList in ApiKeysView, CustomFieldsView, RecurringTicketsView
- Convert boot overlay fade-out from style.opacity to .boot-overlay--fade-out CSS class
- Add .boot-overlay--fade-out rule to dashboard.css

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

views/DashboardView.php

@@ -63,7 +63,7 @@ $nonce = SecurityHeadersMiddleware::getNonce();
                         i++;
                     } else {
                         setTimeout(() => {
-                            bootOverlay.style.opacity = '0';
+                            bootOverlay.classList.add('boot-overlay--fade-out');
                             setTimeout(() => bootOverlay.remove(), 500);
                         }, 500);
                         clearInterval(interval);