Improve readability of progress bars (#16509)

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## Summary

Improve readability of progress bars by drawing the right portions in
dimmed black instead of dimmed green. This makes it much easier to
distinguish from the left (finished) part, which is drawn in green.

Fixes #6908.
<!-- What's the purpose of the change? What does it do, and why? -->

## Test Plan

Tested locally, looks as follows:

<img width="1100" height="731" alt="white"
src="https://github.com/user-attachments/assets/2a396e96-27ef-41ed-8b03-a0de2061af12"
/>
<img width="1100" height="731" alt="black"
src="https://github.com/user-attachments/assets/85d03a3a-a1dc-4389-9e51-fd486e60e067"
/>

crates/uv/src/commands/reporters.rs

@@ -230,7 +230,7 @@ impl ProgressReporter {
             // Ignore spinners, such as for builds.
             if let ProgressBarKind::Numeric { progress, .. } = progress {
                 let template = format!(
-                    "{{msg:{max_len}.dim}} {{bar:30.green/dim}} {{binary_bytes:>7}}/{{binary_total_bytes:7}}"
+                    "{{msg:{max_len}.dim}} {{bar:30.green/black.dim}} {{binary_bytes:>7}}/{{binary_total_bytes:7}}"
                 );
                 progress.set_style(
                     ProgressStyle::with_template(&template)
@@ -252,7 +252,7 @@ impl ProgressReporter {
             progress.set_style(
                 ProgressStyle::with_template(
                     &format!(
-                        "{{msg:{}.dim}} {{bar:30.green/dim}} {{binary_bytes:>7}}/{{binary_total_bytes:7}}", state.max_len
+                        "{{msg:{}.dim}} {{bar:30.green/black.dim}} {{binary_bytes:>7}}/{{binary_total_bytes:7}}", state.max_len
                     ),
                 )
                     .unwrap()