feat: duplicate detection + mark-as-duplicate, lt-toggle preferences in settings

- Dependencies tab: auto-loads potential duplicates via /api/check_duplicates.php
  on first activation; shows 'Mark duplicate' button per result which POSTs to
  ticket_dependencies with type=duplicates and refreshes the dependencies list
- Settings modal: replaced checkboxes with lt-toggle switches for
  notifications_enabled and sound_effects; loads current user prefs on modal open
  and saves via /api/user_preferences.php on SAVE button

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

assets/js/ticket.js

@@ -507,6 +507,7 @@ function showTab(tabName) {
         initializeUploadZone();
     } else if (tabName === 'dependencies') {
         loadDependencies();
+        loadPotentialDuplicates();
     }
 }
 
@@ -531,6 +532,70 @@ function loadDependencies() {
         });
 }
 
+// Load potential duplicates from check_duplicates API and show "Mark as duplicate" buttons
+let _dupsLoaded = false;
+function loadPotentialDuplicates() {
+    if (_dupsLoaded) return;
+    _dupsLoaded = true;
+
+    const frame = document.getElementById('potentialDupsFrame');
+    const list  = document.getElementById('potentialDupsList');
+    if (!frame || !list) return;
+
+    const title = window.ticketData?.title || document.querySelector('.title-input')?.textContent?.trim() || '';
+    if (!title) return;
+
+    lt.api.get('/api/check_duplicates.php?title=' + encodeURIComponent(title))
+        .then(data => {
+            if (!data.success || !data.duplicates || !data.duplicates.length) return;
+
+            // Filter out this ticket itself
+            const thisId = String(window.ticketData.id);
+            const dupes  = data.duplicates.filter(d => String(d.ticket_id) !== thisId);
+            if (!dupes.length) return;
+
+            let html = '<ul class="duplicate-list lt-text-sm">';
+            dupes.forEach(dup => {
+                html += `<li class="lt-flex lt-flex-gap-sm lt-flex-align-center" style="padding:0.3rem 0">
+                    <a href="/ticket/${lt.escHtml(String(dup.ticket_id))}" class="lt-text-cyan lt-text-xs" target="_blank">#${lt.escHtml(String(dup.ticket_id))}</a>
+                    <span style="flex:1;overflow:hidden;text-overflow:ellipsis;white-space:nowrap">${lt.escHtml(dup.title)}</span>
+                    <span class="lt-text-muted lt-text-xs">${lt.escHtml(String(dup.similarity))}% · ${lt.escHtml(dup.status)}</span>
+                    <button type="button" class="lt-btn lt-btn-ghost lt-btn-xs mark-dup-btn"
+                            data-dup-id="${lt.escHtml(String(dup.ticket_id))}"
+                            title="Link this ticket as a duplicate of #${lt.escHtml(String(dup.ticket_id))}">
+                        Mark duplicate
+                    </button>
+                </li>`;
+            });
+            html += '</ul>';
+            list.innerHTML = html;
+            frame.style.display = '';
+
+            list.querySelectorAll('.mark-dup-btn').forEach(btn => {
+                btn.addEventListener('click', () => {
+                    const dupId = btn.dataset.dupId;
+                    const ticketId = window.ticketData.id;
+                    lt.api.post('/api/ticket_dependencies.php', {
+                        ticket_id: ticketId,
+                        depends_on_id: dupId,
+                        dependency_type: 'duplicates'
+                    }).then(res => {
+                        if (res.success) {
+                            btn.textContent = '✓ Linked';
+                            btn.disabled = true;
+                            btn.classList.add('lt-btn-primary');
+                            lt.toast.success('Linked as duplicate of #' + dupId);
+                            loadDependencies();
+                        } else {
+                            lt.toast.error(res.error || 'Failed to link dependency');
+                        }
+                    }).catch(() => lt.toast.error('Network error'));
+                });
+            });
+        })
+        .catch(() => {}); // silent — duplicate check is advisory only
+}
+
 function showDependencyError(message) {
     const dependenciesList = document.getElementById('dependenciesList');
     const dependentsList = document.getElementById('dependentsList');