HTML Email Signature Templates with Variable Support

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

No files, tests, or entry points are named. Start by locating the existing signature handling, administrator settings, send-time user metadata, and API paths; then define the smallest supported template and variable scope. Done means administrators can manage reusable HTML templates, variables resolve for recipients, and the requested preview, mailbox, and deployment behavior is covered.