VolumesViewModel: fire-and-forget cleanup Task in defer block

[AprilNEA]

Problem

VolumesViewModel launches a cleanup Task { } inside a defer block without awaiting it. The Task may be discarded before completion, leaving orphaned Docker volumes.

File: ArcBox/ViewModels/VolumesViewModel.swift:147-154

Fix

Await the cleanup or restructure to ensure the delete completes before returning.

[linear]

ABXD-83