Fix re-rendering of comments when typing in reply box

Extract CommentsSection into a memoized component to prevent
unnecessary re-renders when typing in the reply textbox.

Changes:
- Create new CommentsSection component with memo and useCallback
- Fix useMemo dependencies in comment-list.tsx (remove JSON.stringify)
- Use stable callback for setCommentsPage
- Apply responsive width classes to post container

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>