Fix segments scaling

[zpqrtbnk]

This PR should fix #140.

I have reviewed the code changes.
I have tested locally and it works and fixes the issue.

Warning

I am not a seasoned Windows UI / Rust developer, and so I have used Claude's help in order to diagnose the issue, and then to propose a fix. I have reviewed the code manually, made manual tweaks, and think it makes enough sense.

Summary

See issue #140.

Root cause: positions are correct on average, but each segment and gap has to snap to whole pixels, and the leftover fractions build up unevenly. Especially, at 150%, segments all 15, but gaps switch between 2 and 1. And on a 12–15 px segment, one pixel is about 7–8%, which is easy to see.

What the PR does: Round both the segment and the gap, then let the bar's total width differ slightly from 109 × scale. At 150% that's 15 × 10 + 2 × 9 = 168 px instead of 164. That means changing the layout, because the percentage text is positioned from the same width expression.

Testing

Run on a 150% scaling display, verify that segments are balanced.

Checklist

• I have read CONTRIBUTING.md and CODE_OF_CONDUCT.md.
• The change is focused and contains no credentials or sensitive data.
• I have added or updated relevant documentation and tests, where applicable.
• I have left changelog and version changes to maintainers unless requested.