[IMPROVEMENT] Decouple Avalonia to reduce installation footprint

[Kobi-Blade]

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

Currently, UniGetUI redistributes Avalonia UI within the same package as Classic UI. This results in an installation size of approximately 678MB. For the majority of users who only use one interface, carrying the weight of both frameworks is redundant and inefficient.

I suggest entirely separating the two interfaces:

• Decouple the Avalonia UI, making Avalonia entirely optional.
• Maintain this separation until the team is ready to fully deprecate the Classic UI in favor of Avalonia, rather than merging both during the transition period.

Describe how this improvement could help users

• Lower disk space usage, this is crucial for users like me who run UniGetUI in portable mode on devices with limited storage.
• There is no benefit to bundling two separate UI frameworks when the vast majority of users will only ever use one (those who wish to experiment can still install both).

[mamoreau-devolutions]

this is already improved for the next release: #4719

[Kobi-Blade]

That PR simply merges both, which goes against my suggestion:

• Maintain this separation until the team is ready to fully deprecate the Classic UI in favor of Avalonia, rather than merging both during the transition period.

It doesn't solve the problem of having two UI frameworks in the project, honestly, it's only making it worse.

[Kobi-Blade]

Since it was already merged, is not useful to anyone to continue this discussion.