fix(ui): render AddBookmarkModal using portal to fix z-index stacking

Use React createPortal to render modal directly to document.body, bypassing the sidebar's stacking context (z-index: 1) which was preventing the modal from appearing above other elements

src/components/AddBookmarkModal.tsx

@@ -1,4 +1,5 @@
 import React, { useState, useEffect, useRef } from 'react'
+import { createPortal } from 'react-dom'
 import { FontAwesomeIcon } from '@fortawesome/react-fontawesome'
 import { faTimes, faSpinner } from '@fortawesome/free-solid-svg-icons'
 import IconButton from './IconButton'
@@ -183,7 +184,7 @@ const AddBookmarkModal: React.FC<AddBookmarkModalProps> = ({ onClose, onSave })
     }
   }
 
-  return (
+  return createPortal(
     <div className="modal-overlay" onClick={onClose}>
       <div className="modal-content" onClick={(e) => e.stopPropagation()}>
         <div className="modal-header">
@@ -280,7 +281,8 @@ const AddBookmarkModal: React.FC<AddBookmarkModalProps> = ({ onClose, onSave })
           </div>
         </form>
       </div>
-    </div>
+    </div>,
+    document.body
   )
 }