fix: stack high-usage-warning slider layout, clamp range to 50-90

[coldhighsun]

Summary

• Fixed the Create/Edit Disk dialog's high-usage-warning row so the checkbox and the percent slider/value stack vertically instead of sitting side-by-side in a fixed-width row - the long English checkbox label was pushing the slider/value text off the visible window.
• Narrowed the warning threshold slider (and the corresponding clamp when loading an existing disk profile) from 1-99 to 50-90.

Test plan

• dotnet build src/ManagedDrive.App succeeds
• Manually open Create Disk dialog, verify the slider/value are visible in English and Chinese, and the value stays within 50-90