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/css/base.css

@@ -922,7 +922,8 @@ pre {
 .lt-table-wrap { overflow-x: auto; border: 2px solid var(--terminal-green); }
 
 /* Sortable column header */
-.lt-table th[data-sort] { cursor: pointer; }
+.lt-table th[data-sort],
+th[data-sort-key] { cursor: pointer; }
 .lt-table th[data-sort]:hover { color: var(--terminal-green); text-shadow: var(--glow-green); }
 .lt-table th[data-sort="asc"]::after  { content: ' ▲'; color: var(--terminal-green); }
 .lt-table th[data-sort="desc"]::after { content: ' ▼'; color: var(--terminal-green); }
@@ -1177,6 +1178,7 @@ pre {
 .lt-toast-close::before,
 .lt-toast-close::after { content: ''; }
 .lt-toast-close:hover  { opacity: 1; transform: none; }
+.lt-toast--hiding      { opacity: 0; transition: opacity 0.3s ease; }
 
 /* ----------------------------------------------------------------
    15. TAB NAVIGATION