feat: locking days, if they are submitted and accepted

fixed #76

Backend/models/publicHoliday.go

@@ -19,6 +19,11 @@ type PublicHoliday struct {
 	worktime int8
 }
 
+// IsSubmittedAndAccepted implements IWorkDay.
+func (p *PublicHoliday) IsSubmittedAndAccepted() bool {
+	return true
+}
+
 // IsEmpty implements [IWorkDay].
 func (p *PublicHoliday) IsEmpty() bool {
 	return false