fix(RepoLoadingOverlay): adjust loading overlay position

The loading overlay's position has been updated to account for a new
layout change. This ensures that the overlay is displayed correctly
below the app title bar, improving the user experience during loading.

- Adjusted overlay position to maintain consistency with layout changes

src/lib/components/RepoLoadingOverlay.svelte

@@ -33,7 +33,7 @@
 <style>
   .repo-loading {
     position: fixed;
-    top: var(--app-titlebar-height, 42px);
+    top: calc(var(--app-titlebar-height, 42px) + 56px);
     right: 0;
     bottom: 0;
     left: 0;