⏪️(mail) revert mail upgrade due to unhandled breaking changes

Rollback the mail package upgrade after identifying multiple
breaking changes introduced in v5 that were not fully accounted
for.

Local testing initially missed the issue because the mail Docker
image had not been rebuilt automatically, causing broken emails to
go unnoticed.

src/mail/package.json

@@ -5,7 +5,7 @@
   "type": "module",
   "dependencies": {
     "@html-to/text-cli": "0.5.4",
-    "mjml": "5.0.0"
+    "mjml": "4.18.0"
   },
   "private": true,
   "scripts": {