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>

assets/css/dashboard.css

@@ -2725,6 +2725,7 @@ input[type="checkbox"]:checked {
     justify-content: center;
     transition: opacity 0.5s;
 }
+.boot-overlay--fade-out { opacity: 0; }
 
 #boot-text {
     font-family: var(--font-mono);