Avoid float division in TOTP code

[KamilaBorowska]

This mostly doesn't matter because 64-bit floats can represent 53-bit integers (and currently UNIX timestamps are 32 bits), but still, no reason to work with floats when integers will work fine.

This avoids Year 285618384 problem :).

[egonelbre]

Also, there's a slight bug on M1 processors due to float and uint conversions being platform dependent. https://go.dev/ref/spec#Conversions

On M1 Mac the uint64(float64) conversion can end up returning 0. Here's an example program.

package main

import (
	"math"
	"time"
)

func main() {
	period := uint(30)
	x := time.Time{}
	counter := int64(math.Floor(float64(x.Unix()) / float64(period)))
	counter2 := uint64(math.Floor(float64(x.Unix()) / float64(period)))
	println(counter, counter2)
	// Output: -2071186560 0
}

[KamilaBorowska]

@egonelbre

TOTP algorithm doesn't support timestamps before Unix epoch (time.Time{} is year 1). Doesn't really matter as time moves forwards, so this shouldn't be possible in practice.