Hallo
Probier mal folgendes CSS
.mainheader {
display: flex;
flex-wrap: wrap;
justify-content: space-between;
align-items: flex-end;
}
.mainheader img {
min-width: 1px;
}
.mainheader nav {
width: 100%;
margin-top: 18px;
}
Gruss
MrMurphy