feat(comments): hide every comment mention when COMMENTS_ENABLED=false

The kill-switch previously left comment UI/text visible in several places.
Sweep the whole surface so a comments-off instance shows no trace:

Frontend (main app):
- VirtualFeed grid tile: gate the comment button/count on $commentsEnabled
  (was ungated — the only feed surface still showing it).
- admin stats: hide the "Kommentare" count card.
- UploadSheet "Uploads geschlossen" notice, host ban-modal description, and
  host/admin unban confirmations: drop the "…und kommentieren" wording.
- export page: drop "Kommentaren" from the keepsake description.

Keepsake export (had no concept of the flag):
- export.rs: thread comments_enabled into the exported data (ViewerEvent),
  wired through spawn_export_jobs/recover_exports and their call sites
  (host.rs, main.rs).
- export-viewer: gate comment counts (list + grid) and the lightbox comments
  section; older exports without the field default to enabled (?? true).

Backend still 403s comment writes when disabled (unchanged) — this is the UI
half so stale clients and archives match.

Verified on the running stack (COMMENTS_ENABLED=false): /event reports
comments_enabled=false, a regenerated keepsake embeds "comments_enabled": false
with no comment UI, and the uploads-closed notice renders "…ansehen und liken."

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/routes/export/+page.svelte

@@ -7,6 +7,7 @@
 	import { toastError } from '$lib/toast-store';
 	import { focusTrap } from '$lib/actions/focus-trap';
 	import IconButton from '$lib/components/IconButton.svelte';
+	import { commentsEnabled } from '$lib/event-config-store';
 
 	interface JobStatus {
 		status: 'locked' | 'pending' | 'running' | 'done' | 'failed';
@@ -266,7 +267,8 @@
 					<div class="min-w-0">
 						<h2 class="section-title">HTML-Viewer</h2>
 						<p class="mt-0.5 text-sm text-gray-500 dark:text-gray-400">
-							Schöne Offline-Galerie mit Filterung, Kommentaren und Likes — kein Internet nötig.
+							Schöne Offline-Galerie mit Filterung{$commentsEnabled ? ', Kommentaren' : ''} und Likes
+							— kein Internet nötig.
 						</p>
 						<p
 							class="mt-1 text-xs {status.html.status === 'done'