feat: #3637902 Add a PNG email logo for HTML emails

Issue: https://www.drupal.org/i/3637902

Adds logo-email.png, logo.svg as a 123×73 PNG, for HTML emails. Gmail and Outlook do not show SVG images, so emails that use the site logo show no logo there. 123×73 is the size the site header shows the logo at (7.675rem wide), and the Easy Email Theme template sets no width on the logo. The README says what the file is for. Same approach as Varbase Theme - Bootstrap 5 (#3637288).

The Rightup points the Easy Email Theme logo at this file (#3637905), so this MR should be released first.

Testing

1. On a site with The Rightup, copy logo-email.png into themes/contrib/vartheme_bs5_rightup/ and set easy_email_theme.settings logo.use_default: false, logo.path: themes/contrib/vartheme_bs5_rightup/logo-email.png.
2. Send a password reset email and open it in Mailpit.
3. The logo <img> points to logo-email.png and shows at the same size as the site logo.

AI-Generated: Yes

Checkpoints:

• File an issue
• Addition/Change/Update/Fix
• Testing to ensure no regression
• Automated unit testing coverage
• Automated functional testing coverage
• UX/UI designer responsibilities
• Readability
• Accessibility
• Performance
• Security
• Developer Documentation
• User Guide Documentation
• Reviewed by human
• Code review by maintainers
• Full testing and approval
• Credit contributors
• Review with the product owner
• Release notes snippet
• Release