fix(NcAppNavigation): Restore opaque background on mobile

The mobile navigation slides in over the content area, where the
content wrapper's frosted chrome no longer backs it. The drawer was
appearing transparent and the page underneath bled through. The
overlay now has its own opaque blurred background again.

Signed-off-by: nfebe <fenn25.fn@gmail.com>

Author: nfebe

src/components/NcAppNavigation/NcAppNavigation.vue

@@ -467,6 +467,9 @@ export default {
 	.app-navigation {
 		position: absolute;
 		border-inline-end: 1px solid var(--color-border);
+		background-color: var(--color-main-background-blur, var(--color-main-background));
+		-webkit-backdrop-filter: var(--filter-background-blur, none);
+		backdrop-filter: var(--filter-background-blur, none);
 	}
 }