feat: #3637288 Add a PNG email logo for HTML emails

Issue: https://www.drupal.org/i/3637288

Adds logo-email.png, logo.svg as a 150×30 PNG, for HTML emails. Gmail and Outlook do not show SVG images, so emails that use the site logo show no logo there. It keeps the site logo's displayed size because the Easy Email Theme template sets no width on the logo. The README says what the file is for.

Varbase Starter points the Easy Email Theme logo at this file (#3637289), so this MR should be released first.

Testing

1. On a Varbase 11 site with Varbase Starter, copy logo-email.png into themes/contrib/vartheme_bs5/ and set easy_email_theme.settings logo.use_default: false, logo.path: themes/contrib/vartheme_bs5/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. Mailpit's HTML Check lists no SVG warning.

Remaining tasks

• MR pipeline green. The Cucumber dry-run passed; the install job timed out reaching Launchpad (add-apt-repository), so the e2e job was skipped. Rerun once Launchpad is reachable.

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