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also harden the replacement logic

Add new function to convert nanoseconds into `approximate_total:` for *Generation Info* tooltip.

Browsers and plugins that spoof canvas data produce corrupt images. In attempt to mitigate:

* Add canvasPixelTest() to test a single pixel and test the RGB values
* Test canvasPixelTest() inside generateInitialsImage() and use default `/user.png` if failure detected
* Call canvasPixelTest() directly within settings to avoid setting an invalid image
* Use toast.error() with 10 second autoClose

This reverts commit 9763d885.