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>

backend/src/handlers/host.rs

@@ -302,6 +302,7 @@ pub async fn rebuild_export(
         r.event_id,
         r.event_name,
         r.epoch,
+        state.config.comments_enabled,
         std::time::Duration::ZERO,
         state.pool.clone(),
         state.config.media_path.clone(),
@@ -549,6 +550,7 @@ pub fn start_regen(state: &AppState, regen: crate::services::export::PendingRege
         regen.event_id,
         regen.event_name,
         regen.epoch,
+        state.config.comments_enabled,
         // Debounced: a takedown pass is a burst, and each request retires the last generation. The
         // delay lets superseded workers fail their claim and do zero work instead of each building
         // a full archive. See export::REGEN_DEBOUNCE.
@@ -753,6 +755,7 @@ pub async fn release_gallery(
         event_id,
         event_name,
         epoch,
+        state.config.comments_enabled,
         std::time::Duration::ZERO,
         state.pool.clone(),
         state.config.media_path.clone(),