fix: fix surplus calculation for very big numbers

[shoom3301]

Summary

For some reason, the surplus is wrongly displayed for very big numbers.

Example: https://explorer.cow.fi/orders/0xe05b6a805656fdf2a4c404be0756f360e0297c53d00371da637a44e133676d2e6cd22a1d90eeba4bfd0080ad9f121d4a33b1fda96459d692?tab=overview

Background

bignumber.js wrongly round very small numbers - when you divide something small to something very big

Actual result:

const minimumBuyAmount = new BigNumber('4000000000000000000000000000').multipliedBy(
      (new BigNumber('7152000000'))
        .dividedBy(new BigNumber('4000000000000000000000000000'))
    )
// 7160000000

Expected result:

const minimumBuyAmount = (4000000000000000000000000000) * (7152000000 / 4000000000000000000000000000 )
// 7152000000

To test

The displayed surplus should look the same for any other values:

1. Very small
2. Normal
3. Very big

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
explorer-dev	✅ Ready (Inspect)	Visit Preview	💬 Add feedback

📖 Storybook ↗︎

[github-actions]

Pull Request Test Coverage Report for Build 5220880806

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 61.585%

---

Totals
Change from base Build 5188962808:	0.0%
Covered Lines:	872
Relevant Lines:	1231

---

💛 - Coveralls

[alfetopito]

Didn't quite work

Also the unit tests are broken, probably due to the same reason?

[shoom3301]

@alfetopito sorry, it was an attempt to use Anxo's suggestion. Now it works.
@elena-zh fyi

[elena-zh]

Changes LGTM.
Still, we have an issue with a surplus percentage, but it is reported in #462 and can be addressed later.

[alfetopito]

Nice, looks great now!

One gentle reminder that you should do that for the buySurplus as well