Migrate stacks/stacks.go AskForCluster from survey to huh

[blsmth]

Summary

Migrate the AskForCluster shared helper in stacks/stacks.go from survey to huh.

Details

• Complexity: Medium
• Prompts: 1 Select (cluster picker)
• Shared helper used by database, redis, and app_pipeline stacks
• Uses clusterSelectTransform to map display names to stack names — replace with huh.NewOption(displayName, stackName)
• Changes function signature from response interface{} to *string
• All 3 callers need updating
• Also fixes apppack app create --cluster flag still prompts for cluster selection #115 (skip prompt when --cluster flag provided)

Checklist

• Extract form builder function (pure, no AWS mocks needed)
• Replace survey Select with huh form using typed options
• Update function signature and all 3 callers
• Fix apppack app create --cluster flag still prompts for cluster selection #115: skip prompt when --cluster flag is provided
• Add teatest + ui/uitest tests
• Add VHS recording for visual verification
• Verify non-interactive mode still works