Revert z-index to main layout

Signed-off-by: Alex Andreev <alex.andreev.email@gmail.com>

src/renderer/components/layout/main-layout.module.scss

@@ -11,6 +11,7 @@
   grid-template-rows: [contents] 1fr [footer] auto;
   grid-template-columns: [sidebar] var(--sidebar-width) [contents] 1fr;
   width: 100%;
+  z-index: var(--z-index-base);
   height: 100%;
 }