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/lib/components/UploadSheet.svelte

@@ -6,6 +6,7 @@
 	import CameraCapture from '$lib/components/CameraCapture.svelte';
 	import type { PendingFile } from '$lib/pending-upload-store';
 	import { eventState, uploadsClosed } from '$lib/event-state-store';
+	import { commentsEnabled } from '$lib/event-config-store';
 
 	// Uploads closed (event locked or gallery released) — show a lock notice instead of
 	// the capture options, so a guest can't stage a photo that would just be rejected.
@@ -163,8 +164,9 @@
 			<div class="rounded-xl bg-amber-50 px-5 py-4 text-center dark:bg-amber-950/30">
 				<p class="font-semibold text-amber-800 dark:text-amber-300">Uploads geschlossen</p>
 				<p class="mt-1 text-sm text-amber-700 dark:text-amber-400">
-					Der Host hat die Uploads für dieses Event beendet. Du kannst weiterhin Fotos ansehen,
-					liken und kommentieren.
+					Der Host hat die Uploads für dieses Event beendet. Du kannst weiterhin Fotos ansehen{$commentsEnabled
+						? ', liken und kommentieren'
+						: ' und liken'}.
 				</p>
 			</div>
 			<button onclick={close} class="btn btn-secondary btn-block"> Schließen </button>