Better Alerting When Trying to Exit a Running Conversion

[iuravic]

Is your feature request related to a problem? Please describe.
Currently if the end user tries to close the active tab with the conversion app running, no special warning is displayed, besides the default web browser's "Leave?" alert.

Describe the solution you'd like
Add an explicit warning alert informing the user that it's vital to keep the browser tab open.

Describe alternatives you've considered
In a later release, it would be great to be able pause the conversion with an option to continue running it later. But for now, we just want to make it clear to the user that the ongoing conversion shouldn't be cancelled.