Warning message if a Passkey exists

### Checks

- [x] I have read the Wiki, searched the open issues, and still think this is a new feature.

### Explain the problem clearly and succinctly:

Prevent a user from saving a Passkey that already exists

### Describe the solution you'd like:

Display a warning message indicating that a Passkey with the same Relying Party and Username already exists.
Linked to #2282

### Describe alternatives you've considered:

_No response_

### Additional context:

_No response_

No file or test is named. Start by reading linked issue #2282 and locating the Android passkey save flow; done means attempting to save a passkey with the same Relying Party and Username displays a warning instead of silently saving a duplicate.

Written by the indexing model from the issue text.