style(frontend): remove hardcoded indigo colors in settings components

Replace all manual bg-indigo and text-indigo overrides with standard CSS variables such as bg-primary/10 and text-primary across common settings modules to support theme integration.

frontend/app/(main)/admin/settings/page-client.tsx

@@ -80,7 +80,7 @@ export function AdminSettingsPageClient() {
   if (loading || !user || !user.is_admin) {
     return (
       <div className="flex items-center justify-center min-h-[400px]">
-        <Loader2 className="size-6 animate-spin text-indigo-500" />
+        <Loader2 className="size-6 animate-spin text-primary" />
       </div>
     )
   }