feat: add local MJML renderer using spatie/mjml-php

[MelvinAchterhuis]

Summary

• Extracts MJML rendering into a strategy pattern (MjmlRendererInterface) so the rendering backend is swappable
• Adds LocalMjmlRenderer using spatie/mjml-php for local Node.js-based MJML compilation
• Existing shyim.de API renderer (ApiMjmlRenderer) remains the default — fully backwards compatible
• Configurable via bundle config: mjml_renderer: local to opt-in to local rendering

Configuration

# config/packages/frosh_platform_template_mail.yaml
frosh_platform_template_mail:
    mjml_renderer: local  # default: api

Requires Node.js 16+ and the mjml npm package when using local mode.

Test plan

• Existing API renderer tests pass (migrated to ApiMjmlRendererTest)
• New LocalMjmlRendererTest passes with spatie/mjml-php
• MjmlLoaderTest updated to test file reading and include parsing independently
• Tested end-to-end: switched to local mode, triggered mail send, verified HTML output in Mailpit
• Verified api mode (default) still works identically

🤖 Generated with Claude Code