Feature: Download only option and Trunk SHA verification fix

[HeyMeco]

Description

This PR does two things:

1. Add option to only download the selected image to disk

   • Toggle with checkbox for decompression option
   • Localisation for this feature in all languages supported by imager
   

2. Fix SHA verification for armbian/os trunk images on GitHub by utilising the GitHub API which offers SHA of the artifact

   • Caches fetched api response
   • Before SHA comparison fails as there is no .sha file on dl.armbian.com for the requested Armbian-trunk...

All features have been implemented with Claude Opus 4.5 as a guide and validated in use. Please review carefully to make sure there is no regression

[SuperKali]

Hi @HeyMeco,

I like the idea, but as it currently stands, it might be a bit confusing for users. I think it would be more appropriate to add a toggle in the settings. The download option should also be placed in something separate rather than directly within the device selector, which in my opinion doesn't quite fit—users should be selecting their device there, not downloading images. Of course, it doesn't make sense to add another box on the homepage, but if you have any ideas on where to place that download button in a way that's also aesthetically pleasing, please let me know. In the meantime, if I come up with any ideas, I'll share them here.

Thank you for your contribution to the project!

[SuperKali]

Fix SHA verification for armbian/os trunk images on GitHub by utilising the GitHub API which offers SHA of the artifact

• Caches fetched api response
• Before SHA comparison fails as there is no .sha file on dl.armbian.com for the requested Armbian-trunk...

Can you show me examples where it actually fails?