refactor(posts): improve the post layout - #63

Summary

• Use the same Post component for sample and published posts.
• Place the author and avatar in the header, let the body use the full width, and separate the engagement counts with a subtle divider.
• Give the sample authors stable avatar fallback colors using all eight Ares variants.

Test Plan

• npm run lint
• npm run format:check
• npm run typecheck
• npm run test -- --maxWorkers=4 (1,568 tests passed)
• npm run build

Ares

Does this pull request change Ares? No.