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/admin/ApiKeysView.php

@@ -66,7 +66,7 @@ $nonce = SecurityHeadersMiddleware::getNonce();
             </div>
 
             <!-- New Key Display (hidden by default) -->
-            <div id="newKeyDisplay" class="ascii-frame-inner key-generated-alert" style="display: none;">
+            <div id="newKeyDisplay" class="ascii-frame-inner key-generated-alert is-hidden">
                 <h3 class="admin-section-title">New API Key Generated</h3>
                 <p class="text-danger text-sm mb-1">
                     Copy this key now. You won't be able to see it again!
@@ -195,7 +195,7 @@ $nonce = SecurityHeadersMiddleware::getNonce();
             if (data.success) {
                 // Show the new key
                 document.getElementById('newKeyValue').value = data.api_key;
-                document.getElementById('newKeyDisplay').style.display = 'block';
+                document.getElementById('newKeyDisplay').classList.remove('is-hidden');
                 document.getElementById('keyName').value = '';
 
                 lt.toast.success('API key generated successfully');