HTML Email Signature Templates with Variable Support

[Bytrider]

Is your feature request related to a problem? Please describe.

Nextcloud Mail currently supports only basic, manually maintained email signatures.
There is no support for reusable HTML templates with dynamic user data.

This makes it difficult to:

• Enforce corporate identity standards
• Maintain signatures efficiently for larger organizations

Describe the solution you'd like

Add support for HTML-based email signature templates with variable placeholders, managed by administrators:

• Create or upload .html signature templates
• Reusable templates across users and mailboxes
• Dynamic placeholders that are automatically replaced, e.g.:

Kind regards  
{{displayName}}  
{{jobTitle}}  
{{organisation}}  
Email: {{email}}  
Phone: {{phone}}

Possible variables

• Username
• Display name
• Email address
• Organization / department
• Phone number
• Role or group

The variables should be resolved automatically at send time using user metadata.

Describe alternatives you've considered

• CI-compliant, professional email signatures
• Central maintenance with personalized data
• Scales well for organizations with many users

Additional context

• Per-user signature preview
• Multiple templates per mailbox
• API support for automated deployment

[ChristophWurst]

Makes sense. This is a follow-up to #12224.