Add configurable solid background colour behind health bars

[chrisliebaer]

This adds a solid colour layer behind the health and missing health bars. It's useful when running both bars at low opacity, you get a dark backdrop that makes the bars readable without needing to crank up the bar opacity itself. I use this with class colours at 15% foreground to create a "damage bar" look where the missing health portion pops out. Without the solid background, the low opacity bars look washed out against the game world:

With a black solid background at 70% opacity, the bars sit on a dark base and are properly tinted, muted, but also provide separation from the background.

Default is fully transparent so nothing changes for existing users.

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I tried to keep this as close to the existing code style as possible, same colour picker + opacity slider pattern, same default structure, same update path. But if you'd rather handle this differently, I'm happy to rework it. I'd rather iterate on it than have it sit.