[IMPROVEMENT] Add day-of-week selector for "Check for updates every: 1 week"

[JonathanPitre]

Please confirm these before moving forward.

• I have searched for my feature proposal and have not found a work-in-progress/duplicate/resolved/discarded issue.
• This improvement refers to an existing feature. If you want to suggest a new feature, please use this template.
• This improvement is not a bug. If you want to report a bug, please use this template.

Describe the improvement

In Package update preferences > Update checking, when "Check for updates every: 1 week" is selected, add an option to choose the specific day of the week.

Also if the same could be done for UniGetUI self-updater that would be awesome.

Thank you!

Describe how this improvement could help users

This allows aligning UniGetUI updates with Windows Update schedule for enterprise rollouts.