[FEATURE REQUEST] Scheduled install time for updates

### 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 proposal is a completely new feature. If you want to suggest an improvement or an enhancement, please use [this template](https://github.com/Devolutions/UniGetUI/issues/new?labels=enhancement&projects=&template=enhancement-improvement.yml&title=%5BENHANCEMENT%5D+%28Enter+your+description+here%29).

### Describe the new feature

Currently, UniGetUI offers "Install Available Updates" which installs updates as soon as they're detected. I'd like the option to schedule when updates are installed, rather than having them install immediately. For example, being able to set a specific time (e.g., 3:00 AM) for installations to run.

### Describe how this new feature could help users

This would prevent updates from interrupting workflow during the day. Users could schedule installs during off-hours (overnight, lunch break, etc.) when they're not actively using their computer, similar to how Windows Update allows setting "active hours." It would also help avoid the disruption of multiple UAC/admin prompts popping up when you're trying to work or gaming.

Start by locating the existing "Install Available Updates" flow in UniGetUI and trace how update installation is triggered. The issue provides no file or test names, so first identify the relevant scheduling and installation entry points; done means users can configure a time and updates install then rather than immediately.

Written by the indexing model from the issue text.