Resolves #352

Subtask of #348.

This PR aligns the next/v3 CLI’s --use-preview-props behavior, help text, and tests with main while keeping email check based on caniemail.

Summary of changes

• CLI flags (next/v3)

  • Updated email build and email check to accept the --use-preview-props flag in the same way as on main.
  • Ensured the flag is parsed consistently for both commands and passed into the shared helpers.

• Build / check pipeline wiring

  • Threaded usePreviewProps through the buildTemplates (and related) helpers used by email build so builds honor previewProps when the flag is set.
  • Updated the email check pipeline to pass usePreviewProps to template building/checking so the same previewProps used for builds/previews are also used for checks.
  • Explicitly kept email check using caniemail for compatibility checks (did not change it to use @jsx-email/doiuse-email).

• CLI help and docs

  • Aligned the email build and email check help output on next/v3 with main by:
    • Documenting --use-preview-props as a first-class flag for both commands.
    • Making clear that email check runs caniemail-based checks while still honoring previewProps when requested.
  • Updated related CLI docs to make --use-preview-props discoverable and described consistently across branches.

• Tests

  • Added/updated tests for email build on next/v3 to verify that:
    • --use-preview-props is accepted and correctly wired into the build pipeline.
    • Template rendering during build respects previewProps when the flag is enabled.
  • Added/updated tests for email check on next/v3 that:
    • Confirm --use-preview-props is accepted and forwarded into the checking pipeline.
    • Ensure checks are still performed via caniemail while using the same previewProps as builds/previews.
    • Mirror the intent and coverage of the equivalent tests on main for parity.

Notes

• All changes are targeted at and based on next/v3, not main.
• The choice of caniemail for email check is preserved per the follow-up comments on charlie: head vs next/v3 #348; this PR only aligns how --use-preview-props is exposed, documented, and exercised in tests.