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/mail.go

@@ -5,7 +5,6 @@ import (
 	"fmt"
 	"log/slog"
 	"net/smtp"
-	"spend-sparrow/internal/types"
 )
 
 type Mail interface {
@@ -14,10 +13,10 @@ type Mail interface {
 }
 
 type MailImpl struct {
-	server *types.Settings
+	server *Settings
 }
 
-func NewMail(server *types.Settings) MailImpl {
+func NewMail(server *Settings) MailImpl {
 	return MailImpl{server: server}
 }