Allow users to switch between oldest-first and newest-first comment
ordering via a toggle button in the comments heading. The preference is
persisted as a frontend setting (commentSortOrder) and passed to the API
as the order_by query parameter so pagination works correctly.
When all comments fit on a single page, toggling reverses them
client-side to avoid an extra API call. New comments are inserted at the
correct position based on the active sort order, and in newest-first
mode the view scrolls to the top after adding a comment.
Link-share users fall back to a local (non-persisted) sort order since
they cannot save user settings.
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