feat(backend): add email validator package

Add ValidateEmail and NormalizeGiftCardCode functions with comprehensive tests. Migrate guest.go from inline mail.ParseAddress to validators.ValidateEmail.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/handlers/user/guest.go

@@ -6,7 +6,6 @@ import (
 	"errors"
 	"log"
 	"net/http"
-	"net/mail"
 	"regexp"
 	"strings"
 
@@ -69,10 +68,9 @@ func CreateGuestUserHandler(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	// Validate email format (contains @ and .)
-	_, err := mail.ParseAddress(req.Email)
-	if err != nil {
-		http.Error(w, "invalid email format", http.StatusBadRequest)
+	// Validate email format
+	if err := validators.ValidateEmail(req.Email); err != nil {
+		http.Error(w, err.Error(), http.StatusBadRequest)
 		return
 	}
 
@@ -140,8 +138,8 @@ func CheckEmailHandler(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if _, err := mail.ParseAddress(email); err != nil {
-		http.Error(w, "invalid email format", http.StatusBadRequest)
+	if err := validators.ValidateEmail(email); err != nil {
+		http.Error(w, err.Error(), http.StatusBadRequest)
 		return
 	}