[sddm-theme/lock screen] Adjust login screen and lock screen font sizes
Summary: This patch increases the font size of the username and action button labels for better readability.
Test Plan:
Before:
After:
For reference, level 4:
Reviewers: VDG, Plasma, ngraham
Reviewed By: VDG, ngraham
Subscribers: filipf, ngraham, plasma-devel
Tags: Plasma
Maniphest Tasks: T10325
Differential Revision: https://phabricator.kde.org/D19433