[BUG] Precision Loss from Floating Point Math

[DeveloperAmrit]

[y] I have checked for duplicate issues

Description:
Conversion functions for USD equivalents use native JS parseFloat and multipliers like 1e6. This causes rounding errors for large currency amounts.

How to Reproduce:
Call calculateBcUsdEquivalent with a very large amountFloat (e.g., 10^18 wei equivalent).

Actual Behaviour:
Resulting values lose precision after the 15th significant digit, which is unacceptable for financial software.

Expected Behaviour:
All currency calculations must stay within BigInt or a dedicated library like bignumber.js until the final display stage.

Location: StablePay/djed-sdk/src/helpers.js

[y] I want to work on this issue

[Rav1Chauhan]

assign it to me.