MNTOR-5282 - Swap email logos for dark-mode variants

[codemist]

References:

Jira: MNTOR-5282

Description

Adds dark-mode support for the Monitor and Mozilla logos in the transactional emails. When a recipient's mail client is set to dark mode, the standard logos are hidden and dark-mode-friendly variants are shown in their place. Light mode rendering is unchanged.

The technique used here is a CSS-driven image swap: each logo is rendered twice, once tagged with dm-img-light and once with dm-img-dark. The dark variant is hidden by default, and a prefers-color-scheme: dark media query inverts which one is visible.

Screenshot (if applicable)

How to test

Checklist (Definition of Done)

• Localization strings (if needed) have been added.
• Commits in this PR are minimal and have descriptive commit messages.
• I've added or updated the relevant sections in readme and/or code comments
• I've added a unit test to test for potential regressions of this bug.
• If this PR implements a feature flag or experimentation, I've checked that it still works with the flag both on, and with the flag off.
• If this PR implements a feature flag or experimentation, the Ship Behind Feature Flag status in Jira has been set
• Product Owner accepted the User Story (demo of functionality completed) or waived the privilege.
• All acceptance criteria are met.
• Jira ticket has been updated (if needed) to match changes made during the development process.
• Jira ticket has been updated (if needed) with suggestions for QA when this PR is deployed to stage.

[joeherm]

LGTM- thanks for the tests.