Handle sequential PAM prompts in greeter and locker

[demesm]

After the first PAM response, COSMIC remained in the authenticating state, preventing later PAM prompts from becoming normally interactive.

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Summary

• Reset the authentication UI state when a new interactive PAM prompt arrives.
• Apply the sequential-prompt fix to both the graphical greeter and session locker.

Testing

• Tested on Fedora 44 with COSMIC.
• mfa w/ Password + YubiKey + TOTP succeed through pamtester. Graphical login & unlock work with the fixes.