Fix NOTIFY_EMAIL_FROM plus sign encoding in mailto URLs

URL-encode e.From in mailto query parameters so that + characters
are preserved instead of being decoded as spaces. Fixes #1946

backend/app/notify/email.go

@@ -168,7 +168,7 @@ func (e *Email) buildAndSendMessage(ctx context.Context, req Request, email stri
 				ctx,
 				fmt.Sprintf("mailto:%s?from=%s&unsubscribeLink=%s&subject=%s",
 					email,
-					e.From,
+					url.QueryEscape(e.From),
 					url.QueryEscape(msg.unsubscribeLink),
 					url.QueryEscape(msg.subject),
 				),
@@ -203,7 +203,7 @@ func (e *Email) SendVerification(ctx context.Context, req VerificationRequest) e
 				ctx,
 				fmt.Sprintf("mailto:%s?from=%s&subject=%s",
 					req.Email,
-					e.From,
+					url.QueryEscape(e.From),
 					url.QueryEscape(e.VerificationSubject),
 				),
 				msg,