fix(reacties): can now also reply to a reply (not just top-level comments)
The backend already supported this (resolvedParent flattens to the top-level
thread), but the view only showed a Reply button + form on top-level comments.
Replies now also get a Reply button + inline form (user && canMutate); the reply
lands in the same thread (1 level deep preserved). Existing toggle JS picks up
the new .comment-reply-btn automatically.
Co-Authored-By: Claude <noreply@…>