Support migrating old-format bookmarks

[SnowCait]

Parent: #2282

Depends on: #2307

Goal

Allow users to explicitly copy deprecated old-format bookmarks into standard Bookmarks.

Scope

• Copy both public and private old-format bookmarks.
• Leave the old-format bookmarks unchanged when copying.
• Provide a separate, explicit NIP-09 deletion action for users who no longer need the old-format bookmarks.
• Do not migrate bookmarks automatically.
• Exclude Bookmark sets (kind: 30003); they are tracked in Support NIP-51 bookmark sets #2308.

Acceptance criteria

• Users can explicitly copy public and private old-format bookmarks into standard Bookmarks.
• Copying does not modify the old-format bookmarks.
• Users can explicitly delete the old-format bookmarks through NIP-09 independently of copying.