Remove Pixels: Mul<Pixels, Output = Pixels> impl, add ScaledPixels ops (#27451)

It doesn't make sense to have `Pixels: Mul<Pixels, Output = Pixels>` as
the output should be `Pixels^2` (area), so these impls are removed. All
code where these impls were used are improved by instead multiplying by
`f32` or `usize`.

Also adds math op impls that are present for `Pixels` but absent for
`ScaledPixels`. Adds missing `Mul<Pixels> for usize` to both.

Release Notes:

- N/A

crates/file_finder/src/file_finder.rs

@@ -1067,7 +1067,7 @@ fn full_path_budget(
     small_em: Pixels,
     max_width: Pixels,
 ) -> usize {
-    ((px(max_width / px(0.8)) - px(file_name.len() as f32) * normal_em) / small_em) as usize
+    (((max_width / 0.8) - file_name.len() * normal_em) / small_em) as usize
 }
 
 impl PickerDelegate for FileFinderDelegate {