Fix install button position

Add `position: relative` to the parent `div` that owns the scrollbar, so that Install button positions itself relative to it and not to the whole document.

Fixes a bug where button would get rendered on top of a scrollbar.

src/components/intro/style.scss

@@ -33,6 +33,7 @@
   height: 100%;
   box-sizing: border-box;
   overscroll-behavior: contain;
+  position: relative;
 }
 
 .logo-container {