Spaghetti in backgrounds form, implement lazy loading

[unclamped]

While reviewing #12 and coming up with my own solution, I stumbled across file https://github.com/RiiConnect24/RiiTag-Next/blob/master/src/lib/constants/forms/backgrounds.js. It seems like the background selector in the RiiTag control panel is directly loading all backgrounds at once, from a predefined list. This could be improved in the future to recursively search the backgrounds folder, which would not only clean up the code a bit, but would also allow for seamless display of new backgrounds, such as custom ones as indicated in #12.