[Feature Request] Mail rules: keep mail header (from/to/subject) in PDF when layout is "HTML only"

Description

When a mail rule uses the default PDF layout "Text, then HTML", the generated PDF contains three parts:

1. The mail header information (from, to, subject, etc.)
2. A raw text representation of the mail
3. An HTML rendering of the mail

When the PDF layout is changed to "HTML only", the raw text part is omitted, as expected. However, the header information is omitted as well, so the resulting PDF contains only the rendered HTML body.

Any of the following would solve this:
• In "HTML only" mode, still render the header block above the HTML body, so the layout differs from the default only by the missing raw text part.
• Add a separate option on the mail rule, e.g. "Include mail header in PDF", that works independently of the layout choice.

Based on #14657

Other

_No response_