[Bookmarks] Add drag-and-drop reordering for bookmarks and folders

[shadowbipnode]

Add drag-and-drop support for bookmarks and folders.

Goals:

• drag bookmarks inside the bookmarks bar
• drag folders inside the bookmarks bar
• reorder items visually
• move bookmarks into folders
• move items between root/bookmarks bar/folders
• persist sort_order and parent_id
• support safe drop targets
• avoid accidental deletion or duplication

This should make bookmark management feel closer to mainstream browsers.