timer-face: Use "title-1" style class for the "Select Duration" title

This title is using the "timer-header" style class, but it resembles the "large-title" style class from libadwaita, which is deprecated.

The libadwaita docs instruct to use "title-1" instead of "large-title": https://gnome.pages.gitlab.gnome.org/libadwaita/doc/main/style-classes.html#large-title

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