fix: replace vulnerable email regex with safe helper function

- Use isValidEmail from helpers instead of inline regex
- Fixes SonarQube Security Hotspot for DoS via backtracking

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/register/page.tsx

@@ -7,6 +7,7 @@ import { registerSchema, RegisterFormData } from "@/lib/validations";
 import Link from "next/link";
 import TeaIconsBackground from "@/components/TeaIconsBackground";
 import { ROUTES, API_BASE_URL, API_ENDPOINTS } from "@/utils/constants";
+import { isValidEmail } from "@/utils/helpers";
 import ReCAPTCHA from "react-google-recaptcha";
 
 export default function RegisterPage() {
@@ -42,7 +43,7 @@ export default function RegisterPage() {
 
   // Vérifier si l'email existe déjà
   const checkEmail = async (email: string) => {
-    if (!email || !/^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email)) {
+    if (!email || !isValidEmail(email)) {
       setEmailStatus({ checking: false, exists: null, valid: null, message: '' });
       return;
     }