Fix amount preview for scanned receipts

[mertd]

To ensure the formatter works as expected, I also set up testing. The test cases ended up taking a lot more time than the fix itself, but I wanted to make sure not to introduce any other unexpected formatting bug.

As a mostly unrelated change, I needed to change the GPT model used as I was getting depreciation errors during development.

Fixes #87 @RayBB
Fixes #140 @Phlipde
Fixes #121 @webzit

Do the changes from #81 still work as expected @justcallmelarry?

[mertd]

@scastiel I remember now why I left this branch untouched for a long time: I couldn't test if this actually fixed the issue without being able to upload to a public S3 instance. However, when I picked this up again yesterday, I assumed I had it tested manually already and just needed to finish up the automatic util tests. 😅

Could you please trigger a preview deployment with Vercel? If not, I will have to pick this up again later for some testing with a public S3 instance.

[scastiel]

Hey @mertd, I just tested and it works like a charm!

I'm taking the liberty to assume your PR is "ready for review" so I can merge it (I don't have much time to dedicate to Spliit, so who knows when I have a PR-merging moment in the future 😉).

Thanks very much! (And thanks for introducing Jest too, might be useful!)