fix: enhance email notification formatting with HTML for improved readability

backend/internal/services/notification_service.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"html"
 	"net"
 	"net/http"
 	neturl "net/url"
@@ -283,24 +284,24 @@ func (s *NotificationService) dispatchEmail(ctx context.Context, p models.Notifi
 	safeTitle := sanitizeForEmail(title)
 	safeMessage := sanitizeForEmail(message)
 	subject := fmt.Sprintf("[Charon Alert] %s", safeTitle)
-	// Build a plain-text body; MailService will convert this to safe HTML and
-	// perform additional sanitization before sending.
 	var bodyBuilder strings.Builder
 	if safeTitle != "" {
-		bodyBuilder.WriteString(safeTitle)
+		bodyBuilder.WriteString("<strong>")
+		bodyBuilder.WriteString(html.EscapeString(safeTitle))
+		bodyBuilder.WriteString("</strong>")
 	}
 	if safeMessage != "" {
 		if bodyBuilder.Len() > 0 {
-			bodyBuilder.WriteString("\n\n")
+			bodyBuilder.WriteString("<br>")
 		}
-		bodyBuilder.WriteString(safeMessage)
+		bodyBuilder.WriteString(html.EscapeString(safeMessage))
 	}
-	plainBody := bodyBuilder.String()
+	htmlBody := bodyBuilder.String()
 
 	timeoutCtx, cancel := context.WithTimeout(ctx, 30*time.Second)
 	defer cancel()
 
-	if err := s.mailService.SendEmail(timeoutCtx, recipients, subject, plainBody); err != nil {
+	if err := s.mailService.SendEmail(timeoutCtx, recipients, subject, htmlBody); err != nil {
 		logger.Log().WithError(err).WithField("provider", util.SanitizeForLog(p.Name)).Error("Failed to send email notification")
 	}
 }