feat(widget): add option for label at center of progress bar

[pitchaya-s]

This PR adds support for a new center_label option in the progress_bar config:

progress_bar:
  enabled: true
  center_label: "\uf4bc"

• Accepts available {icon} from widgets (center_label: "{icon}").
• Label can be styled using .progress-circle, with support for dynamic classes.

.pomodoro-widget .progress-circle.work {
    color: red
}