fix(api): ensure balance is captured as number of cents when user account created

[jan-molak]

Hi Team!

I noticed that when a user account is created via a POST request with balance specified in the payload correctly as number, isUserValidator converts this value to string.
This results in the createUser saving incorrect data type to database.json, which then prevents the NavDrawer from formatting the value, and causes the entire UI to crash.

This patch ensures that whenever a user account is created programmatically, the balance is correctly stored as number.

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[jennifer-shehane]

@jan-molak Thanks for the fix and adding a test 😄