feat: extract into remaining packages

There has been a cyclic dependency.
transaction
	-> treasure_chest
	-> transaction_recurring
	-> transaction

This has been temporarily solved by moving the GenerateTransactions
function into the transaction package. In the future, this function has
to be rewritten to use a proper Service insteas of direct DB access or
replaced with a different system entirely.

internal/core/format.go

@@ -0,0 +1,36 @@
+package core
+
+import (
+	"fmt"
+	"strings"
+)
+
+func FormatEuros(balance int64) string {
+	prefix := ""
+	if balance < 0 {
+		prefix = "- "
+		balance = -balance
+	}
+
+	n := float64(balance) / 100
+	s := fmt.Sprintf("%.2f", n) // "1234567.89"
+
+	parts := strings.Split(s, ".")
+	intPart := parts[0]
+	fracPart := parts[1]
+
+	var result strings.Builder
+	numberOfSeperators := len(intPart) % 3
+	if numberOfSeperators == 0 {
+		result.WriteString(intPart)
+	} else {
+		for i := range intPart {
+			if i > 0 && (i-numberOfSeperators)%3 == 0 {
+				result.WriteString(",")
+			}
+			result.WriteByte(intPart[i])
+		}
+	}
+
+	return prefix + result.String() + "." + fracPart + " €"
+}