The settings endpoint returned the raw key to any caller, and the
frontend used that response to pre-fill the input field, which also
meant saving with an untouched field would silently resubmit the
plaintext key. /gemini_key now reports only whether a key is set and
a masked preview; the UI shows that as a placeholder and only updates
the stored key when the field is actually filled in.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
