Warn before quitting while an image/batch is generating

It can feel pretty painful to accidentally quit during a 75 step image. It'd be great if there was a confirmation dialogue that appeared during an active batch/image to prevent that.

There should be no dialogue shown while the app is idle, of course.

This should be pretty straightforward, with a [`window.onbeforeunload`](https://developer.mozilla.org/en-US/docs/Web/API/Window/beforeunload_event#examples) event, and then toggling behavior via [`is_input_avail`](https://github.com/divamgupta/diffusionbee-stable-diffusion-ui/blob/2abd542179d80fca806fa2900d13cd31117ea290/electron_app/src/StableDiffusion.vue#L36) status.