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

### Please confirm these before moving forward.

- [x] I have searched for my feature proposal and have not found a work-in-progress/duplicate/resolved/discarded issue.
- [x] This improvement refers to an existing feature. If you want to suggest a new feature, please use [this template](https://github.com/Devolutions/UniGetUI/issues/new?labels=new-feature&projects=&template=feature-request.yml&title=%5BFEATURE+REQUEST%5D+%28Enter+your+description+here%29).
- [x] This improvement is not a bug. If you want to report a bug, please use [this template](https://github.com/Devolutions/UniGetUI/issues/new?labels=bug&projects=&template=bug-issue.yml&title=%5BBUG%5D+%28Enter+your+description+here%29).

### 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.