Rename a length's abs component to px

`dp` is coming, and then `abs` says which of the two it is not. The
component has always been a pixel count, so the name that admits it is
the one that leaves room for a second unit beside it.

Mechanical: the field on `Len` and `UiScalar`, their constructors,
`to_abs`/`get_abs`, the matching WGSL struct member and the locals
composing it. Field order and types are unchanged, so the `Pod` layout
the shader reads is the same bytes. `f32::abs` is untouched.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/widget/image.rs

@@ -8,11 +8,11 @@ pub struct Image {
 impl Widget for Image {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         painter.primitive(&self.handle);
-        Size::abs(self.handle.size())
+        Size::px(self.handle.size())
     }
 
     fn size_hint(&self, axis: Axis) -> Option<Len> {
-        Some(Len::abs(self.handle.size().axis(axis)))
+        Some(Len::px(self.handle.size().axis(axis)))
     }
 
     fn on_resize(&self, _: Axis) -> OnResize {