fix(lightbox): hide comments toggle when allow_comments=false (#518)

@Rekoo-PS reported the MessageSquare comment button stayed visible in
the lightbox toolbar even when guest comments were disabled. Same
class of bug as #513 (per-photo Like button missing the master
gate) but on a different control.

The Like and Rating buttons in the lightbox toolbar gate correctly:
  feedbackEnabled && feedbackSettings?.allow_likes
  feedbackEnabled && feedbackSettings?.allow_ratings

The MessageSquare button only checked feedbackEnabled. Since likes
and ratings already have their own inline buttons in the same
toolbar, this third button is effectively the "open comments panel"
affordance — its badge counts comments, its tooltip mentions
comments. When comments are off it has nothing meaningful to do.

Add allow_comments to the local feedbackSettings type (the backend
already returns it via galleryFeedback.js:33) and gate the button on
feedbackEnabled && feedbackSettings?.allow_comments.

Refs: #518

frontend/src/components/gallery/PhotoLightbox.tsx

@@ -77,6 +77,7 @@ export const PhotoLightbox: React.FC<PhotoLightboxProps> = ({
     feedback_enabled?: boolean;
     allow_likes?: boolean;
     allow_ratings?: boolean;
+    allow_comments?: boolean;
     require_name_email?: boolean;
   } | null>(null);
   const [myLiked, setMyLiked] = useState<boolean>(false);
@@ -684,8 +685,12 @@ export const PhotoLightbox: React.FC<PhotoLightboxProps> = ({
               </div>
             )}
             
-            {/* Feedback button with indicator */}
+            {/* Feedback button with indicator. Gated on allow_comments
-            {feedbackEnabled && (
+                because likes/ratings already have their own dedicated
+                toolbar buttons above — this MessageSquare button only
+                opens the comments panel, so it has nothing to do when
+                comments are off (#518). */}
+            {feedbackEnabled && feedbackSettings?.allow_comments && (
               <button
                 onClick={() => {
                   setShowFeedback(!showFeedback);