Background Image with Color Fill Option

[floork]

Description

I would like to propose an enhancement to Hyprlock that allows users to set a background image along with an optional color fill for any uncovered areas. This feature would be beneficial in cases where the selected image does not fully cover the screen, eliminating the need for unwanted zooming or stretching. By enabling users to define a color to fill the remaining space, we can create a more aesthetically pleasing and customizable lock screen experience.

see #461