Subtask of #348. Align next/v3 CLI use of --use-preview-props with main by wiring the flag through email build and email check, updating CLI help and tests, and ensuring email check continues to use caniemail (not @jsx-email/doiuse-email) per the follow up comment.

Target branch

All work for this issue should be implemented against the next/v3 branch.

• Branch from origin/next/v3.
• Open pull requests with base set to next/v3 (not main).

Context

main and next/v3 both support usePreviewProps in the build pipeline, but they differ in how the CLI exposes and documents it:

• On main, --use-preview-props is clearly documented and is wired through both email build and email check so checks can run against the same previewProps as builds/previews.
• On next/v3, email check intentionally remains based on caniemail and should stay that way, but the handling of --use-preview-props and the associated help text are not yet aligned with main.

The goal of this issue is to make the next/v3 CLI feel consistent with main wherever --use-preview-props is involved, without changing the underlying choice of caniemail.

Implementation notes

When working this issue:

1. Use charlie: head vs next/v3 #348 as the reference for how --use-preview-props flows through buildTemplates and other shared helpers on main.
2. Update the next/v3 implementations of email build and email check so they:
   • Accept the --use-preview-props flag in the same way as main.
   • Wire the flag through to template building/checking so previewProps are honored consistently.
3. Align the CLI help output and docs on next/v3 so --use-preview-props is discoverable and described the same way as on main, while explicitly keeping email check on caniemail.
4. Add or update tests to cover the --use-preview-props behavior for both build and check on next/v3, mirroring the intent of the tests on main.