Log error when sendEmailHandler fails to send an email

Send Reminder failures were silently swallowed, only surfacing in the
HTTP response and never in the application log, unlike the GM
notification email flows.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

cmd/web/sendemailhandler.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"log"
 	"net/http"
 	"strings"
 
@@ -29,6 +30,7 @@ func sendEmailHandler(w http.ResponseWriter, r *http.Request) {
 	// log.Default().Printf("got email: To: %s, From: %s, Subject: %q, Body: %q", to, from, subject, body)
 	err := email.SendEmail(e)
 	if err != nil {
+		log.Default().Printf("ERROR sending email: %s", err)
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
 	}