feat(stacks): migrate app_pipeline.go from survey to huh

[blsmth]

Migrates the last remaining survey-using file to huh. Completes the migration chain started with #116.

Changes

• stacks/app_pipeline.go — all 10+ survey prompts converted to huh forms (helper builders, AskForDatabase/Redis/SES, AskQuestions, WarnIfDataLoss)
• Deleted databaseSelectTransform and redisSelectTransform — replaced by huh typed options
• cmd/modify.go — updated to use the renamed form builders
• stacks/app_pipeline_test.go — teatest coverage for the new form builders

Silent bug fix

AskForRedisStack previously compared against databaseStackNameTmpl instead of redisStackNameTmpl when pre-selecting the current redis stack — so the pre-selection never matched. Fixed in this migration.

Closes #123

[blsmth]

Code Review

Nice — this closes out the survey→huh migration cleanly. Flow is preserved, conditionals still fire in the right order, and the typed huh.Option[string] replacement for databaseSelectTransform/redisSelectTransform is much easier to reason about than the reflection-based callbacks.

Bug fix (already noted in PR body — good)

AskForRedisStack had a pre-existing bug where it compared against databaseStackNameTmpl instead of redisStackNameTmpl — so the pre-selection never matched. Correctly fixed here.

Test coverage gaps (worth follow-up, not blocking)

• TestAppDataLossConfirmForm_Confirm / _Reject don't actually assert anything (_ = confirmedPtr). Given that "default = No = abort destructive op" is the whole point of preserving this flow, at least one test should verify *confirmedPtr == false on default.
• TestAppPrivateS3Form_SelectYes uses SelectNth(tm, 0) which equals SelectFirst, so with defaultEnabled=false it actually lands on "no". Misleading name, no assertion — drop or fix.
• Missing coverage for "pre-existing selection" on AppDatabaseStackForm/AppRedisStackForm — verify .Selected(true) on a non-first option is honored.

Nits

• AskForDatabase — verbose gets the same value in both branches; pull it above the if.
• AppDatabaseStackForm/AppRedisStackForm pre-seed selected := options[0].Value rather than the defaultClass pattern used by the instance class forms. Works either way, but inconsistent.
• strings.Split(*domainsPtr, "\n") produces [""] for empty input — matches old survey behavior, not a regression, but easy cleanup as follow-up.

Safe to merge. Test gaps can land as a follow-up PR.