fix: reject invalid volume values

[benrosiers]

Summary

Reject invalid --volume values before starting playback.

Why

The README documents --volume as a value between 0 and 1, but the previous validation only checked numeric range comparisons. Non-numeric values such as banana become NaN, which does not fail < 0 or > 1 checks.

That could allow an invalid value to reach the playback layer and be passed to mpv.

Change

• Parse the volume option once.
• Reject empty, non-finite, below-range, and above-range values.
• Reuse the parsed numeric value when starting playback.

Validation

• npm run build
• npm run lint