Create/Pull operations don't set lastError — Toast shows stale errors

[AprilNEA]

Problem

lastError is only set by start/stop/delete operations. Create and pull operations silently log errors but don't surface them:

• ContainersViewModel.createContainer() — returns nil on failure, no lastError
• ImagesViewModel.pullImage() — returns false, no lastError
• NetworksViewModel.createNetwork() — returns error string instead of setting lastError
• VolumesViewModel.createVolume() — same pattern

Users see no feedback when create/pull fails. Toast may display a stale error from a previous operation.

Fix

Set lastError = nil at start and lastError = ArcBoxClient.userMessage(for: error) in catch blocks for all create/pull operations.

[linear]

ABXD-79