Accessibility pass: ARIA roles, label associations, CSS class migrations

- Add role=dialog/aria-modal/aria-labelledby to all 12 modal overlays (JS + PHP)
- Add aria-label="Close" to all 14 modal close buttons
- Add full ARIA combobox pattern to @mention autocomplete (listbox, option, aria-selected, aria-expanded)
- Add for= attributes to admin filter form labels (AuditLog, UserActivity, ApiKeys)
- Remove dead closeOnAdvancedSearchBackdropClick() from advanced-search.js

CSS/JS style cleanup:
- Move .ascii-banner static styles from JS inline to CSS class; add .ascii-banner--glow
- Add .ascii-banner-cursor, .loading-overlay--hiding, .has-overlay, tr[data-clickable]
- Add .animate-fadein/.animate-fadeout/.comment--deleting to ticket.css
- Add .lt-toast--hiding to base.css; remove opacity/transition inline JS
- Remove redundant cursor:pointer JS (already in th{} CSS rule)
- Remove trailing space in lt-select class attributes

Bug fixes:
- base.js: boot overlay opacity inline style was overriding .fade-out class opacity via
  specificity (1000 vs 20), preventing the fade-out animation — removed
- ascii-banner.js: cursor used blink-caret (border-color only) instead of blink-cursor
  (opacity-based), so the █ cursor never actually blinked — fixed

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

assets/js/advanced-search.js

@@ -19,14 +19,6 @@ function closeAdvancedSearch() {
     lt.modal.close('advancedSearchModal');
 }
 
-// Close modal when clicking on backdrop
-function closeOnAdvancedSearchBackdropClick(event) {
-    const modal = document.getElementById('advancedSearchModal');
-    if (event.target === modal) {
-        closeAdvancedSearch();
-    }
-}
-
 // Load users for dropdown
 async function loadUsersForSearch() {
     try {