[FEATURE REQUEST] Option to download all installers first, then run installations

[klinki]

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 proposal is a completely new feature. If you want to suggest an improvement or an enhancement, please use this template.

Describe the new feature

Currently, UniGetUI installs each selected package by downloading and installing them sequentially. This feature would provide an option to download all required installers for the selected packages first, then run all the installations after all downloads are complete.

Describe how this new feature could help users

• Enables offline/portable installation scenarios (download now, install later).
• Gives the user more control and improves transparency during batch operations.
• Could speed things up - currently I have some issues with my PC and about 50% of updates fail. I have to wait a lot of time before the failure (usually there are some network timeouts for some dependencies etc. so I have to wait for the timeout) and I have to confirm UAC for each package - download of a package doesn't start until previous package is installed/fails. Which means everything is slowed down by waiting for user interaction for UAC and I need to watch the computer all the time for UAC dialogs. Of course this would not disappear by this feature, but at least user wouldn't have to wait for all package downloads. Also the networking issues could be pre-determined and some packages could fail early by not being able to download their dependencies.