Fix/14 day streak achievement

[Suprita736]

📝 Pull Request Description

Related Issue

Closes #2290

Summary

Fix the unreachable "14-Day Streak" achievement by adding the missing 14-day entry to the existing streak milestone configuration.

The milestone progression is now:

3 → 7 → 14 → 30

A regression test was also added to verify that all four streak milestones unlock at their appropriate thresholds.

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Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature
• ♻️ Refactoring
• 📝 Documentation update
• 🎨 UI/UX improvement
• 🔥 Other(please describe) ______

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How Has This Been Tested?

• Added regression coverage for the streak milestone progression.
• Verified the 3-day milestone.
• Verified the 7-day milestone.
• Verified the 14-day "14-Day Streak" milestone.
• Verified the 30-day milestone remains functional.
• Ran the relevant streak tracker unit tests successfully.

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Screenshots (if applicable)

Not applicable — this is a backend streak achievement logic fix.

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Checklist

• My code follows the project's guidelines
• I have tested my changes
• I have updated documentation where necessary
• I have linked the related issue
• My changes do not introduce new warnings or errors

[github-actions]

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📥 Commits

Reviewing files that changed from the base of the PR and between 420348a and ea3ce40.

📒 Files selected for processing (11)

• backend/controllers/flashcardController.js
• backend/controllers/googleCalendarController.js
• backend/controllers/userSheetProgressController.js
• backend/models/User.js
• backend/routes/googleCalendarRoutes.js
• backend/server.js
• backend/tests/googleCalendarController.unit.test.js
• backend/tests/interviewExperienceController.unit.test.js
• backend/tests/streakTracker.unit.test.js
• backend/tests/userSheetProgressController.unit.test.js
• backend/utils/streakTracker.js

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[github-actions]

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