fix: guard ProgressBar/Meter percentage against NaN when min equals max

[mvanhorn]

Summary

ProgressBar and Meter no longer emit NaN% when minValue equals maxValue.

Why this matters

Reported in #10042: when minValue === maxValue, the percentage calculation (value - minValue) / (maxValue - minValue) divides by zero and produces NaN, which surfaced both in the rendered width and the aria-valuetext percent label. The fix treats a zero range as 0% and supplies a 0% value label so assistive tech reads a real value.

Changes

• Guard the percentage against a zero range in both ProgressBar and Meter.
• When the range is zero and no explicit valueLabel is set, format 0 with the percent formatter so the label is 0% instead of NaN%.

Testing

Added ProgressBar and Meter tests for the minValue === maxValue case.

Fixes #10042

AI was used for assistance.

[snowystinger]

Thanks for the PR. Which of these cases have you tried out and what were the results? #10042 (comment)