Fix popup file pickers rendering an empty explorer

The cover-page rule added for the auth windows forced `width: 100%` on
every `.window-cover-page .window-body`, including the popup file,
directory, and save pickers. That body is an item-container with
`overflow: auto`, so it establishes its own formatting context and may
not overlap the floated `.window-sidebar`. Unable to shrink to fit, it
dropped below the sidebar, pushing the file list and the Cancel/Open bar
outside the 700x400 popup and leaving the page background showing
through as grey.

`.window-body-filedialog { width: auto !important }` was meant to cover
this but loses on specificity, one class against two. Scope the auth
rule with `:not(.window-filedialog)` instead; that class is only set for
the three dialog kinds, so no auth or session cover page changes.

src/gui/src/css/style.css

@@ -1277,7 +1277,11 @@ span.header-sort-icon img {
     padding: 0 !important;
 }
 
-.window-cover-page .window-body {
+/* Auth/dialog cover pages fill the viewport. File dialogs are excluded: their
+   body is an item-container sitting beside a floated sidebar, and because
+   `overflow: auto` makes it its own formatting context, a forced 100% width
+   can't shrink to fit and drops below the sidebar instead. */
+.window-cover-page:not(.window-filedialog) .window-body {
     height: 100% !important;
     width: 100% !important;
     padding: 0 !important;