feat(screenshots): list without --locale should default or enumerate available locales

Summary

asc screenshots list missing_required on --locale: 92 distinct installs (5.x, 14 days). Agents frequently do not know the locale set before listing.

Proposed fix

Either default --locale to the version's primary locale (printing the choice on stderr), or, when omitted, list screenshots across all localizations of the selected version with a locale column/field. Prefer the second if the output shape can stay additive; otherwise the first. In the error path, list the locales that exist for the version.

Acceptance criteria

• cmdtest for omitted --locale behavior and for the enumerated error when the version has no localizations
• JSON contract remains additive