Fix: shadowed currentstreak

[Saubhagya1621]

🛠️ Related Issue

Closes: #1451

📌 Description

Fixes a variable-shadowing bug in completeLesson where the streak used to calculate the XP multiplier was different from the streak actually persisted to the database. This could cause a user's XP bonus to be based on one streak value while their saved/displayed currentStreak reflected a different one.

✨ Changes Made

• Removed the inner const currentStreak = getLearningStreak(events) block that shadowed the outer, already-computed currentStreak (date-diff based).
• XP multiplier now reuses the single outer currentStreak, so the reward and the persisted streak can no longer diverge.
• No other logic, routes, or behavior changed — kept the diff minimal.

📷 Screenshots

N/A - backend logic fix, no UI change

🧪 Type of Change

• Bug fix
• New feature
• UI/UX improvement
• Documentation update

✔️ Checklist

• My code follows project style guidelines
• I have tested my changes
• I have linked the related issue
• No unnecessary files are included

🚨 Notes for Contributors

• Please ensure PR is linked to an issue before submission
• Incomplete PRs may not be reviewed

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