feat: implement reaction picker with emoji-only popup

[mrakiyaaa]

Summary

Implemented post reactions in the feed:

• Clicking the Like button now shows a popup with 6 emojis.
• Users can select one emoji to react to the post.
• The selected emoji is shown next to the like count below the post.
• Reaction counts are stored in MongoDB and synced to frontend.
• Reaction popup now hides all numbers for a cleaner UI (emoji-only).

Improvements

• Reactions persist after refresh.
• Current user's selected reaction is visually highlighted.
• Reaction count is calculated dynamically.
• Updated Poster.jsx and API config accordingly.

Screenshot

Closes #<issue_number_if_any>

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