Feat: Add Duplicate option to thread popover

Author: multipleof4

src/parts/modals.html

@@ -1,6 +1,7 @@
 <div id="threadPopover" class="menu-card hidden">
   <button data-action="pin" class="menu-item"><i data-lucide="pin" class="h-4 w-4"></i><span>Pin to top</span></button>
   <button data-action="rename" class="menu-item"><i data-lucide="edit-3" class="h-4 w-4"></i><span>Rename</span></button>
+  <button data-action="duplicate" class="menu-item"><i data-lucide="copy" class="h-4 w-4"></i><span>Duplicate</span></button>
   <button data-action="export" class="menu-item"><i data-lucide="download" class="h-4 w-4"></i><span>Export thread (.json)</span></button>
   <button data-action="delete" class="menu-item text-red-600"><i data-lucide="trash-2" class="h-4 w-4"></i><span>Delete</span></button>
   <button data-action="count_tokens" class="menu-item"><i data-lucide="hash" class="h-4 w-4"></i><span>Count tokens (approx.)</span></button>