Fix smartcard/PIN authentication UX issues

Andreas Lundqvistrequested to merge
alun/kscreenlocker:fix/smartcard-auth-ux into master

Three related fixes for smartcard authentication:

  1. pamauthenticator.cpp: Emit busyChanged(false) after succeeded()/failed() instead of before. Previously busyChanged(false) was emitted before succeeded(), causing QML to reset the UI (selectAll, resetFocus) before Qt.quit() was called, resulting in a visible flash on successful unlock.

  2. fallbacktheme/Greeter.qml: Show PAM prompt as password field placeholder. When PAM requests a secret (e.g. 'PIN for alun:' during smartcard auth), use that text as the placeholder instead of always showing 'Password'. Falls back to translated 'Password' when no PAM prompt is set.

  3. fallbacktheme/Greeter.qml: Prevent UI reset on successful authentication. onBusyChanged was resetting the UI even on successful auth, causing a visible flash before unlock completes. Guard with authSucceeded flag to skip reset on success.

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