[Bookmarks] Add native empty-space context menu for bookmarks bar

[shadowbipnode]

Implement a fully native context menu for empty space in the bookmarks bar.

Goal:

• right click empty bookmarks bar
• show native menu
• actions:
  • New Folder
  • Import Bookmarks
  • Bookmark Current Page
• avoid direct accidental folder creation
• avoid React popup menus
• keep behavior consistent with desktop browsers

Current status:
The + button works and bookmark/folder context menus work, but empty-space right click still needs a clean native menu action architecture.