`common.GetDeployingAppImageName` runs `VerifyImage` and errors out when
no image has been built for the app. A freshly-created app has no image
yet, so the preview could never render for never-deployed apps. Add an
`AllowMissingImage` flag to `BuildOptions` that lets BuildAppChart
substitute a placeholder image string in that case; CommandPreview sets
it. The deploy path still passes `BuildOptions{}`, so missing-image
remains a hard error there.