[Bug]: Streak milestone toasts never fire due to property mismatch

[Suprita736]

Bug Description

Description

Streak milestone notifications are not being triggered after flashcard reviews or sheet-progress saves because recordActivity() now returns the property newlyUnlockedMilestones, while the affected controllers still read the old property name newlyUnlocked.

The affected files are:

• backend/utils/streakTracker.js
• backend/controllers/flashcardController.js
• backend/controllers/userSheetProgressController.js

recordActivity() returns:

{ user, newlyUnlockedMilestones }

### Steps to Reproduce

```markdown
1. Log in to the application.
2. Use the application until the next streak milestone is reached.
3. Complete a flashcard review that records streak activity.
4. Observe the response/data returned from the flashcard review flow.
5. Check whether the newly unlocked streak milestone notification/toast appears.
6. Alternatively, save sheet progress while crossing a streak milestone.
7. Observe whether the streak milestone toast is displayed.
8. Compare the value passed as `newlyUnlockedAchievements` with the `newlyUnlockedMilestones` value returned by `recordActivity()`.

### Expected Behavior

When a user crosses a streak milestone while reviewing a flashcard or saving sheet progress, the newly unlocked milestone should be included in the response as `newlyUnlockedAchievements`.

The frontend should then receive the milestone information and display the corresponding streak milestone toast.

For example:

```js
{
  newlyUnlockedAchievements: [...]
}

### Actual Behavior

When a streak milestone is reached through a flashcard review or sheet-progress save, the milestone data is not correctly propagated to the frontend.

`recordActivity()` returns `newlyUnlockedMilestones`, but the affected controllers still read the old `newlyUnlocked` property. As a result, `newlyUnlockedAchievements` is empty or undefined in the response.

The frontend therefore does not receive the newly unlocked milestone and no streak milestone toast is displayed.

This affects:
- Flashcard reviews
- Sheet-progress saves
- The retry path for sheet-progress saves

The session flow is not affected because `sessionController.js` uses the updated streak handling correctly.

### Severity

Medium

### Screenshots / Screen Recording

_No response_

### Browser

Chrome 

### Operating System

Windows 11

### Additional Context

## Root Cause

`recordActivity()` was changed to return:

```js
{
  user,
  newlyUnlockedMilestones
}

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[Suprita736]

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