Add automated HTML and MJML export regression fixtures

[112-stack]

Context

Campaign Forge exports editable campaigns to responsive HTML and MJML, but contributors currently have no lightweight regression suite for the generated markup.

Goal

Add deterministic fixtures that make export changes safer to review.

Suggested scope

• Create at least three representative campaign fixtures: single-column, multi-column, and image/button content.
• Export each fixture to HTML and MJML.
• Add a small Node-based test command that checks required structure and prevents empty or malformed output.
• Normalize generated IDs or timestamps so results are deterministic.
• Document how to run and update the fixtures.

Acceptance criteria

• Tests run with one documented command.
• Both HTML and MJML exports are covered.
• A deliberately broken export causes the test to fail.
• Fixtures contain no external credentials, tracking IDs, or copyrighted assets.
• README or CONTRIBUTING documentation is updated.

Please comment with a proposed approach before starting if the export functions require refactoring.