Improve Share SearchBox

[alperozturk96]

In scenarios such as:

A recipient trying to re-share a file with the original owner.

Attempting to create a duplicate share with a user who already has access.

Instead of blocking the action with an error or hiding the user from the search results, show a message in the search dropdown like:

Already shared with this user

This approach provides a more positive and informative user experience, replacing error messages with clear feedback.

[jancborchardt]

For comparison, here is what is shown for a Github org when someone is already a member. Nice subline indicator and disabled entry, we can do the same:

This would also be useful in desktop, iOS and web, FYI @tobiasKaminsky @sorbaugh