Centralized, Admin-Managed Email Signatures

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

Currently, email signatures in Nextcloud Mail are fully managed by individual users.
Administrators have no way to centrally define or enforce signatures.

This leads to:

• Inconsistent external communication
• Violations of corporate identity (CI) guidelines
• Increased support and configuration effort

This is especially problematic in organizations with compliance or branding requirements.

Describe the solution you'd like

Introduce a centralized email signature management feature at the admin level that allows:

• Managing signatures:

  • per user
  • per user group
  • per mailbox
  • global default

• Editing signatures in the admin settings

• Controlling user permissions:

  • Make signatures read-only (locked)
  • Allow or disallow user overrides

• Clear precedence rules (e.g. mailbox > user > global)

Describe alternatives you've considered

• Consistent and professional email communication
• Reduced user misconfiguration
• Lower administrative overhead
• Better support for organizational use cases

Additional context

• Signature preview in admin settings
• Multiple signatures per user or mailbox
• Signature selection based on sender address

No files or tests are named in the issue. Start by locating the existing user-managed signature settings and the admin-settings entry point, then clarify precedence, locking, overrides, and multiple-signature behavior before defining the implementation scope and acceptance tests.