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/ticket.css

@@ -466,6 +466,50 @@ textarea[data-field="description"]:not(:disabled)::after {
 }
 
 /* Form Elements */
+/* Helper text below form fields */
+.form-hint {
+    color: var(--terminal-green);
+    font-family: var(--font-mono);
+    font-size: 0.85rem;
+    margin-top: 0.5rem;
+}
+
+.form-hint-warning {
+    color: var(--terminal-amber);
+    font-family: var(--font-mono);
+    font-size: 0.85rem;
+    margin-top: 0.5rem;
+}
+
+/* Visibility group checkbox row */
+.visibility-groups-list {
+    display: flex;
+    flex-wrap: wrap;
+    gap: 0.75rem;
+    margin-top: 0.5rem;
+}
+
+.group-checkbox-label {
+    display: flex;
+    align-items: center;
+    gap: 0.5rem;
+    cursor: pointer;
+}
+
+/* Duplicate warning box and visibility groups (JS-toggled, need margin when visible) */
+#duplicateWarning {
+    margin-top: 1rem;
+}
+
+#visibilityGroupsContainer {
+    margin-top: 1rem;
+}
+
+/* Duplicate found heading */
+.duplicate-heading {
+    margin-bottom: 0.5rem;
+}
+
 .detail-group {
     margin-bottom: 30px;
     padding: 15px;
@@ -944,6 +988,10 @@ textarea.editable {
     }
 }
 
+.animate-fadein  { animation: fadeIn 0.3s ease; }
+.animate-fadeout { animation: fadeIn 0.2s ease reverse; }
+.comment--deleting { opacity: 0; transform: translateX(-20px); transition: opacity 0.3s, transform 0.3s; }
+
 .reply-form-container .reply-header {
     display: flex;
     justify-content: space-between;