Fix unfocused split color balance (#3066)

After upgrading Ghostty to the latest release, b92fb2d9, the unfocused
split color was quite different:

<img width="931" alt="image"
src="https://github.com/user-attachments/assets/9ff21db8-0280-416f-8119-2b0d1f08efc4"
/>

This was surprising because I don't set `unfocused-split-fill` so it
should use the background color.

I traced this to 298d6194f4 where the blue
and green components are divided by 255 but not the red.

Although I'm sure this should be easy to reproduce, the relevant bits of
my configuration file are included below:

```
theme = catppuccin-frappe

unfocused-split-opacity = 0.95
```

This fixes #3065.

macos/Sources/Ghostty/Ghostty.Config.swift

@@ -355,7 +355,7 @@ extension Ghostty {
             }
 
             return .init(
-                red: Double(color.r),
+                red: Double(color.r) / 255,
                 green: Double(color.g) / 255,
                 blue: Double(color.b) / 255
             )