fix: preserve custom Zstd level checkbox state in edit dialog

[coldhighsun]

Summary

• Editing a disk with a custom Zstd compression level set showed the checkbox unchecked on reopen, even though DiskOptions.CustomZstdLevel and the compression preset persisted correctly.
• Root cause: CreateDiskDialog's edit constructor set CustomZstdLevelBox.IsChecked = true before calling UpdateCompressionLevelState(). The Checked handler fires synchronously and immediately unchecked the box because CompressionLevelRow.IsEnabled was still false (stale from the base constructor, before ImagePathBox.Text was populated).
• Fix: call UpdateCompressionLevelState() right after setting ImagePathBox/CompressionLevelBox, before setting CustomZstdLevelBox.IsChecked, so the row is already enabled when the checkbox event fires.

Test plan

• dotnet build
• Manually: create a disk with an image path, edit it, enable custom Zstd level with a value (e.g. 15), save.
• Reopen edit dialog and confirm the custom checkbox is checked, the value shows 15, and the preset combo box is disabled.
• Confirm read-only / no-image-path disks still correctly disable/clear the custom checkbox (no regression).