Fix/streak milestone toast property

[Suprita736]

📝 Pull Request Description

Related Issue

Closes #2288

Summary

Fix streak milestone notifications that were not being propagated after flashcard reviews and sheet-progress saves.

recordActivity() returns newlyUnlockedMilestones, but the affected controllers were still reading the old newlyUnlocked property. This caused newlyUnlockedAchievements to be empty or undefined even when a milestone was unlocked.

This PR:

• Updates the flashcard review flow to use newlyUnlockedMilestones.
• Updates the sheet-progress flow and retry path to use newlyUnlockedMilestones.
• Adds regression test coverage for the corrected milestone propagation.

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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/updated regression tests for streak milestone propagation.
• Verified that newly unlocked milestones are correctly passed as newlyUnlockedAchievements.
• Verified the sheet-progress retry path uses the updated property.
• Ran the relevant backend unit tests successfully.

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Screenshots (if applicable)

Not applicable — this is a backend data-flow 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 76ef7fb.

📒 Files selected for processing (9)

• 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/userSheetProgressController.unit.test.js

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