fix(smtp): preserve <br> line breaks in HTML emails

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).

server/smtp_server.go

@@ -33,6 +33,7 @@ var (
 var (
 	onlySpacesRegex          = regexp.MustCompile(`(?m)^\s+$`)
 	consecutiveNewLinesRegex = regexp.MustCompile(`\n{3,}`)
+	htmlLineBreakRegex       = regexp.MustCompile(`(?i)<br\s*/?>`)
 )
 
 const (
@@ -327,6 +328,9 @@ func readHTMLMailBody(reader io.Reader, transferEncoding string) (string, error)
 	if err != nil {
 		return "", err
 	}
+	// Convert <br> tags to newlines before stripping HTML, so that line breaks
+	// in HTML emails (e.g. from Synology DSM, and other appliances) are preserved.
+	body = htmlLineBreakRegex.ReplaceAllString(body, "\n")
 	stripped := bluemonday.
 		StrictPolicy().
 		AddSpaceWhenStrippingTag(true).