Position:fixed for .global-dropzone-overlay

If the user tried dropping a file onto the paperless-ngx UI, but the page itself had scrolled down a bit, the overlay would have scrolled together with the page.

This commit makes the overlay fixed to the viewport, independent from the scroll position.

This one-word commit was done directly through the GitHub web interface.

src-ui/src/styles.scss

@@ -468,7 +468,7 @@ table.table {
 }
 
 .global-dropzone-overlay {
-  position: absolute;
+  position: fixed;
   top: 0;
   right: 0;
   bottom: 0;