Fix directory preview head width (#1691)

The head was set to `100%`, which meant that it only was as wide as the
viewport, rather than its contents, which can overflow. Now, the width
is `fit-content`, which allows it to overflow with its contents so that
the background and border extend the full width.

Author: esimkowitz

frontend/app/view/preview/directorypreview.scss

@@ -31,7 +31,7 @@
             position: sticky;
             top: 0;
             z-index: 10;
-            width: 100%;
+            width: fit-content;
             border-bottom: 1px solid var(--border-color);
 
             .dir-table-head-row {