Reapply "Add missing admin action files and navigation links"

This reverts commit 4d515bc400.

src/app/admin/import/effects/loading.tsx

@@ -0,0 +1,14 @@
+// src/app/[locale]/admin/import/effects/loading.tsx
+export default function Loading() {
+  return (
+    <div className="grid grid-cols-2 md:grid-cols-4 lg:grid-cols-6 gap-3">
+      {Array.from({ length: 18 }).map((_, i) => (
+        <div key={`effect-skeleton-${i}`} className="rounded-lg border bg-card p-3 animate-pulse">
+          <div className="h-12 w-12 bg-muted rounded mx-auto mb-2" />
+          <div className="h-4 bg-muted rounded w-3/4 mx-auto mb-1" />
+          <div className="h-7 bg-muted rounded" />
+        </div>
+      ))}
+    </div>
+  )
+}