refactor(css): layout-Layer wrappen (Hintergrund-Ebenen, Header, Board-Grid-Wrapper)

src/css/main.css

@@ -449,6 +449,7 @@ html, body {
 }
 }
 
+@layer layout {
 .bg-layer {
   position: fixed; inset: 0; z-index: 0;
   background-size: cover; background-position: center;
@@ -535,6 +536,7 @@ html, body {
   padding: 110px 40px 40px;
   min-height: 100vh;
 }
+}
 
 .board {
   width: var(--board-width);