fix: show total comment count on post pages

[saurabhhhcodes]

Closes #1000

Summary

• display the post-level comment total instead of only the currently loaded page
• keep the total synchronized when top-level comments are added or deleted

Validation

• git diff --check
• npx tsc --noEmit
• npx eslint components/feed/CommentsSection.tsx 'app/main/post/[postId]/page.tsx'\n- npm test -- --run tests/Feed.test.tsx

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