Fix canplay listener and interval accumulation in doUseCamera()

[severe77]

Summary

This PR fixes accumulation of "canplay" event listeners and setInterval
loops in doUseCamera() inside js/utils/utils.js.

Problem

Each call to doUseCamera() attached a new anonymous "canplay" listener
and started a new interval when isVideo was true.

Since the listener reference was not preserved, previously attached
listeners could not be removed. As a result:

• Multiple "canplay" handlers accumulated
• Multiple setInterval(draw, 100) loops could run simultaneously
• Repeated camera activations caused unnecessary memory and CPU usage

Solution

This change:

• Stores the "canplay" handler in CameraManager.canPlayHandler
• Removes any existing handler before attaching a new one
• Tracks the active interval in CameraManager.intervalId
• Clears any existing interval before starting a new one

The original logic and behavior are preserved. No refactoring or structural changes were introduced.

Notes

This PR focuses only on preventing listener and interval accumulation.

[github-actions]

✅ All Jest tests passed! This PR is ready to merge.

[vanshika2720]

@severe77 Properly stores and removes the canplay listener and clears existing intervals, preventing accumulation without changing original behavior.

optional suggestion (not blocking): after clearInterval, you could set CameraManager.intervalId = null to make the state explicit.

[github-actions]

✅ All Jest tests passed! This PR is ready to merge.

[severe77]

good eye 👁️
thanks for the suggestion, @vanshika2720

[severe77]

@walterbender the PR is ready for review. Please have a look when you get a chance.