feat: (ussd) add support for float amount values

• removes all int castings
• rounding down to a precision of 2 dp for higher precision values
• for values with 2 dp, no changes except for str -> float casting

examples inputs and their respective conversions @willruddick :

• 2 -> 2.0
• 2.5 -> 2.5
• 2.55 -> 2.55
• 2.559 -> 2.55
• 2.5599 -> 2.55
• 3.9999999999999999999999999 -> rejected