fix: hide collaborator emails from non-owners (#2160)

* feat: Hide collaborator emails from non-owners in shared lists

Implemented privacy protection for collaborator emails in shared lists.
Non-owners (viewers and editors) can no longer see email addresses of
the list owner or other collaborators. Only the list owner can view
all email addresses.

Changes:
- Modified List.getCollaborators() to return empty strings for emails
  when the requester is not the owner
- Updated ManageCollaboratorsModal UI to conditionally display email
  fields only when they are not empty
- Added comprehensive test to verify email privacy for non-owners while
  ensuring owners can still see all emails

This follows existing privacy patterns in the codebase (similar to how
pending invitation names are masked as "Pending User").

* make the email field nullable

* fix tests

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Co-authored-by: Claude <noreply@anthropic.com>

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