HTML-only emails (e.g. from Synology DSM 7.3 Task Scheduler) use <br>
tags for line breaks. The existing implementation passed the raw HTML
body to bluemonday with AddSpaceWhenStrippingTag(true), which replaced
every tag including <br> with a space, causing all content to appear
on a single unreadable line.
Fix: convert <br> tags to newlines before stripping HTML, so line break
semantics are preserved in the notification body.
Resolves the gap noted in #690 ("very sub-par" HTML support).
