gpui: Fix default colors blue, red, green to match in CSS default colors (#20851)

Release Notes:

- N/A

---

This change to let the default colors to 100% match with CSS default
colors.

And update the methods to as `const`.

Here is an example:

<img width="338" alt="image"
src="https://github.com/user-attachments/assets/dd17b46a-3ad4-4122-8dca-e800644c75b0">

https://codepen.io/huacnlee/pen/ZEgNXJZ

But the before version for example blue: `h: 0.6 * 360 = 216`, but we
expected `240`, `240 / 360 = 0.666666666`, so the before version are
lose the precision. (Here is a test tool: https://hslpicker.com/#0000FF)

## After Update

```bash
cargo run -p gpui --example hello_world
```

<img width="612" alt="image"
src="https://github.com/user-attachments/assets/97d479d8-9c71-4be3-95e0-09af45fe47e2">