[Feature]: Add sign-up button and page

[dmueller-dev]

Problem Statement

Users cannot currently sign up by themselves.

Proposed Solution / Workflow

• Add a sign-up page and menu item
• Add setting to admin backend to enable or disable self-sign-up
• Require new users to confirm their email address (for both self-registered users and those added by the admin)

Alternatives Considered

No response

Additional Context

No response

Checklist

• I have searched existing issues to verify this feature has not already been requested.