Use platform-appropriate semantic colors on visionOS

This replaces many of the opaque colors with semantic colors. visionOS will automatically apply vibrancy to the semantic system colors, which makes them more legible on glass backgrounds and makes VLC feel more tailored to the platform.

One caveat is that toHex will not work for system semantic colors, since they're not always 1:1 with RGB colors; I added a fallback path that forces the dark theme for the web view. I also disabled the theme picker on visionOS because there's no support for light/dark theme on visionOS in general.

Before

After