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/RecurringTicketsView.php

@@ -135,7 +135,7 @@ $nonce = SecurityHeadersMiddleware::getNonce();
                             <option value="monthly">Monthly</option>
                         </select>
                     </div>
-                    <div class="setting-row" id="schedule_day_row" style="display: none;">
+                    <div class="setting-row is-hidden" id="schedule_day_row">
                         <label for="schedule_day">Schedule Day</label>
                         <select id="schedule_day" name="schedule_day"></select>
                     </div>
@@ -251,15 +251,15 @@ $nonce = SecurityHeadersMiddleware::getNonce();
             daySelect.innerHTML = '';
 
             if (type === 'daily') {
-                dayRow.style.display = 'none';
+                dayRow.classList.add('is-hidden');
             } else if (type === 'weekly') {
-                dayRow.style.display = 'flex';
+                dayRow.classList.remove('is-hidden');
                 const days = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday'];
                 days.forEach((day, i) => {
                     daySelect.innerHTML += `<option value="${i + 1}">${day}</option>`;
                 });
             } else if (type === 'monthly') {
-                dayRow.style.display = 'flex';
+                dayRow.classList.remove('is-hidden');
                 for (let i = 1; i <= 28; i++) {
                     daySelect.innerHTML += `<option value="${i}">Day ${i}</option>`;
                 }