feat: add recently viewed posts backend and dashboard integration

[AYUSH-P-SINGH]

Pull Request Description

Summary

This PR adds support for tracking recently viewed posts for authenticated users. Whenever a user opens the comments section of a post from the dashboard, the application records that post in the user's recentlyViewed history.

The backend maintains a unique, ordered list of recently viewed posts in Firestore, ensuring duplicate entries are moved to the top and the history never exceeds 10 posts.

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What This PR Does

Backend

• Added a new POST endpoint:

POST /api/posts/view

The endpoint:

• Accepts userId and postId.
• Validates that the referenced post exists.
• Retrieves the user's existing recentlyViewed history.
• Removes any previous occurrence of the viewed post.
• Inserts the post at the beginning of the list.
• Limits the history to the most recent 10 unique posts.
• Updates the user's Firestore document using a merge operation.

Frontend

Updated the dashboard feed to automatically track recently viewed posts.

When a user expands a post's comments section (toggleComments), the frontend asynchronously calls the /api/posts/view endpoint without affecting the user experience.

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Implementation Details

New

• app/api/posts/view/route.js

  • Implements the recently viewed posts API.
  • Validates requests.
  • Updates the user's recentlyViewed array in Firestore.

Modified

• app/page.js

  • Integrates recently viewed tracking into the comments toggle workflow.

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Testing

Backend

• ✅ Verified that the API correctly stores recently viewed posts.
• ✅ Confirmed duplicate post views move the post to the top instead of creating duplicate entries.
• ✅ Verified the history is capped at 10 unique posts.
• ✅ Confirmed empty histories are handled correctly.
• ✅ Verified invalid or non-existent posts are rejected appropriately.

Project Validation

• ✅ npm run check:structure
• ✅ npm run build

Both completed successfully.

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Type of Change

• ✅ ✨ New feature (non-breaking change)

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Checklist

• ✅ My code follows the project's coding standards.
• ✅ I have performed a self-review of my code.
• ✅ My changes introduce no new warnings or errors.
• ✅ The feature has been tested locally.
• ✅ Existing functionality remains unaffected.

FIXES #149

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