style(repo-loading-overlay): adjust overlay position and backdrop

Remove the extra vertical offset so the loading overlay aligns with the
app titlebar. Update the backdrop color mix and increase blur to improve
contrast and perceived depth while the overlay is visible.

- Align overlay using the titlebar height variable instead of adding a
  fixed offset
- Use a denser color mix and stronger blur for a clearer backdrop effect

src/lib/components/RepoLoadingOverlay.svelte

@@ -33,7 +33,7 @@
 <style>
   .repo-loading {
     position: fixed;
-    top: calc(var(--app-titlebar-height, 42px) + 56px);
+    top: var(--app-titlebar-height, 42px);
     right: 0;
     bottom: 0;
     left: 0;
@@ -47,8 +47,8 @@
         color-mix(in srgb, var(--color-accent) 13%, transparent),
         transparent 38%
       ),
-      var(--app-dialog-backdrop);
-    backdrop-filter: blur(5px);
+      color-mix(in srgb, var(--app-dialog-bg) 78%, #05070a 22%);
+    backdrop-filter: blur(9px);
     animation: overlay-in 180ms ease;
   }