KDE lockscreen silently hangs after authd password prompt

When the KDE lockscreen (`kscreenlocker_greet`) is used with authd, entering a password and pressing Enter (or the submit button) does nothing. The lockscreen neither unlocks nor shows an error. The session hangs indefinitely.

## Steps to reproduce

1. Install and configure authd.
2. Log in as an authd-managed user
3. Allow the session to lock (KDE lockscreen appears).
4. Type the local cached password and press Enter or click the submit button.

## Expected behaviour

The lockscreen authenticates and the session unlocks (or shows an error on failure).

## Actual behaviour

Nothing happens. The password field clears, no error is shown, and no unlock occurs. The screen remains locked indefinitely.

## Environment

- Ubuntu 26.04
- KDE Plasma 6 / kscreenlocker 6.6.5

Reproduce the failure with the KDE lockscreen entry point, kscreenlocker_greet, using the stated authd setup and cached password. Trace the authentication result between kscreenlocker_greet and authd; done means a valid password unlocks the session and an invalid one shows an error instead of hanging.

Written by the indexing model from the issue text.