Replace the single day picker in the "Weekly" frequency with a multi-select
so users can schedule on any combination of weekdays (e.g. Mon/Wed/Fri)
in addition to the existing "Weekdays" Mon-Fri preset.
The backend already accepts any day-of-week list in the cron expression,
so this is a frontend-only change. Relabels the tab to "Days" to reflect
the new behavior.