fix(mail): German spacing for invoice draft template

Separate paragraphs with blank lines; mailbox signature uses Synapse MXID.

cmd/oo/mails.go

@@ -254,16 +254,19 @@ für das Service package (OS image inkl. Backup, Monitoring, RustDesk,
 gemeinsamem Datenträger und Netzwerkzugriff).
 
 Zahlungsziel: 14 Tage netto.
-Steuerhinweis: siehe Notizen auf der Rechnung (Reverse Charge / § 13b UStG).
+
+Steuerhinweis: siehe Notizen auf der Rechnung
+(Reverse Charge / § 13b UStG).
 
 Mit freundlichen Grüßen
+
 Example Author`, number, cost)
 			}
 			draft, err := c.SaveMailDraft(cmd.Context(), onlyoffice.SaveMailDraftParams{
 				From:    from,
 				To:      to,
 				Subject: subject,
-				Body:    onlyoffice.PlainTextToMailHTML(body),
+				Body:    onlyoffice.MailHTMLWithBlankParagraphs(onlyoffice.PlainTextToMailHTML(body)),
 			})
 			if err != nil {
 				return err

mails.go

@@ -231,6 +231,16 @@ func PlainTextToMailHTML(s string) string {
 	return b.String()
 }
 
+// MailHTMLWithBlankParagraphs inserts empty paragraphs between blocks
+// (German Geschäftsbrief-style vertical spacing in OnlyOffice Mail).
+func MailHTMLWithBlankParagraphs(html string) string {
+	html = strings.TrimSpace(html)
+	if html == "" {
+		return ""
+	}
+	return strings.ReplaceAll(html, "</p>\n<p>", "</p>\n<p>&nbsp;</p>\n<p>")
+}
+
 // Int64FromMap coerces OnlyOffice numeric id fields (float64/int/string) to int64.
 func Int64FromMap(m map[string]any, key string) int64 {
 	switch v := m[key].(type) {

mails_test.go

@@ -83,6 +83,10 @@ func TestPlainTextToMailHTML(t *testing.T) {
 	if PlainTextToMailHTML(html) != html {
 		t.Fatalf("html passthrough failed")
 	}
+	spaced := MailHTMLWithBlankParagraphs("<p>A</p>\n<p>B</p>")
+	if spaced != "<p>A</p>\n<p>&nbsp;</p>\n<p>B</p>" {
+		t.Fatalf("spaced: %q", spaced)
+	}
 }
 
 func TestInt64FromMap(t *testing.T) {