feat[#352]: Support specifying custom images

[muhdsalm]

Closes #352

Not tested in a VM, but the finals and ABRoot dump seem to line up properly.

[taukakao]

Please add screenshots of the changed GUI here.

[muhdsalm]

Icons are broken on my system :frowning_face:

[taukakao]

Maybe you're missing the gnome-icon-theme package or something.

But anyway, I think the warning could be more descriptive. Something like: "The author of the image has complete control over your system. Only install images of people you trust."

Also, "if you don't know what you're doing" sounds a bit rude. Maybe "Only enable this if you trust the author of the image"

[muhdsalm]

In its current form, it would've been seen by everybody, hence I wanted to make sure no one would touch it unless they knew what they were doing.

But now that we've decided to move it into its own section, it does make sense to make the warning more descriptive than "don't touch!"

[muhdsalm]

Video of the new format:

https://github.com/user-attachments/assets/9124b3ee-d44b-416e-8b34-ceba5e8b64c5

[taukakao]

if the custom-image toggle is off:

* just have the default image url from recipe as default input_entry

* probably not show the custom-image row in summary or point the value to default row?

@Muqtxdir I think these are addressed now.

[taukakao]

Also, I would recommend waiting with merging this until Monday.

It could cause issues that we can better deal with after the release.

[Muqtxdir]

minor nit: let's merge it after post release not immediately to avoid confusion

[taukakao]

@muhdsalm If you want to merge this now then I'm fine with it.