feat: introduce html mails

[bjoern-m]

Description

This PR introduces support for HTML email rendering and delivery alongside plain text emails. The changes ensure that both plain text and HTML versions of passcode and notification emails are generated and sent, improving email formatting and readability across different clients.

Implementation

• Passcode Email Handling

  • Refactored SendPasscodeResult to separate BodyPlain and BodyHTML.
  • Updated SendPasscode to generate both plain text and HTML email bodies.
  • Modified ReSendPasscode and SendPasscode hooks to handle BodyPlain and BodyHTML separately.

• Email Service Enhancements

  • SendEmail now accepts an additional htmlBody parameter and includes an alternative HTML part.
  • Introduced RenderBodyPlain and RenderBodyHTML methods for rendering text and HTML templates separately.

• Template System Updates

  • Refactored email template handling to distinguish .txt.tmpl (plain text) and .html.tmpl (HTML).
  • Added layout.html.tmpl for consistent HTML email structure.
  • Introduced new HTML versions of existing notification emails (e.g., email_verification.html.tmpl).

• Handler Updates

  • Updated PasscodeHandler to render both plain text and HTML emails.
  • Ensured that webhook notifications include both BodyPlain and Body (HTML).

Todos

• Test email rendering across various email clients to ensure compatibility.