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">

crates/gpui/examples/hello_world.rs

@@ -8,8 +8,10 @@ impl Render for HelloWorld {
     fn render(&mut self, _cx: &mut ViewContext<Self>) -> impl IntoElement {
         div()
             .flex()
-            .bg(rgb(0x2e7d32))
-            .size(Length::Definite(Pixels(300.0).into()))
+            .flex_col()
+            .gap_3()
+            .bg(rgb(0x505050))
+            .size(Length::Definite(Pixels(500.0).into()))
             .justify_center()
             .items_center()
             .shadow_lg()
@@ -18,12 +20,23 @@ impl Render for HelloWorld {
             .text_xl()
             .text_color(rgb(0xffffff))
             .child(format!("Hello, {}!", &self.text))
+            .child(
+                div()
+                    .flex()
+                    .gap_2()
+                    .child(div().size_8().bg(gpui::red()))
+                    .child(div().size_8().bg(gpui::green()))
+                    .child(div().size_8().bg(gpui::blue()))
+                    .child(div().size_8().bg(gpui::yellow()))
+                    .child(div().size_8().bg(gpui::black()))
+                    .child(div().size_8().bg(gpui::white())),
+            )
     }
 }
 
 fn main() {
     App::new().run(|cx: &mut AppContext| {
-        let bounds = Bounds::centered(None, size(px(300.0), px(300.0)), cx);
+        let bounds = Bounds::centered(None, size(px(500.), px(500.0)), cx);
         cx.open_window(
             WindowOptions {
                 window_bounds: Some(WindowBounds::Windowed(bounds)),