feat: Add Study Streak Tracker and Achievement Badges (Fixes #520)

[anushkagupta200615-jpg]

Resolves #520

This pull request introduces a gamified Study Streak Tracker and Achievement Badges system to improve user engagement and study consistency over time.

Changes Included:

• Backend (server/models/User.js & server/routes/auth.js):
  • Updated the User schema to track currentStreak, longestStreak, lastStudyDate, and earned badges.
  • Added a new /study-streak API endpoint that safely increments the streak logic based on 24-hour intervals and awards milestone badges (3-Day, 7-Day, 30-Day).
  • Fixed pre-existing syntax errors and duplicate module imports in server/index.js that were crashing the server.
• Frontend (client/src/pages/Profile.js & Profile.css):
  • Overhauled the Profile dashboard to prominently display a "Streak & Achievements" card.
  • Implemented dynamic fire icons that track the current and longest streaks.
  • Added a "Log Study Session" action button that communicates with the backend.
  • Built a responsive badge grid to display earned achievements.
  • Fixed an unclosed CSS block in Navbar.css and removed lingering git merge conflict markers that were failing compilation.

Testing Instructions:

• Ensure MongoDB is running locally.
• Navigate to the Profile page and click "Log Study Session".
• Verify the streak increments properly and restricts multiple clicks on the same day.

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