chore: remove Letterspace newsletter integration and all LETTERSPACE_* variables

packages/frontend/src/routes/videos/[slug]/+page.svelte

@@ -16,7 +16,6 @@ import { formatVideoDuration, getUserInitials } from "$lib/utils";
 import { invalidateAll } from "$app/navigation";
 import { toast } from "svelte-sonner";
 import { createCommentForVideo, likeVideo, unlikeVideo, recordVideoPlay, updateVideoPlay } from "$lib/services";
-import NewsletterSignup from "$lib/components/newsletter-signup/newsletter-signup-widget.svelte";
 import SharingPopupButton from "$lib/components/sharing-popup/sharing-popup-button.svelte";
 
 const { data } = $props();
@@ -539,8 +538,6 @@ let showPlayer = $state(false);
           </CardContent>
         </Card> -->
 
-        <!-- <NewsletterSignup /> -->
-
         <!-- Back to Videos -->
         <Button
           variant="outline"