feat(auth): Convert login system to use email as username

- Backend login now accepts 'email' field (with backward compatibility)
- User creation (signup, invitation, customer) uses email as username
- Frontend login form updated with email input and validation
- Updated test users to use email addresses as usernames
- Updated all translation files (en, es, fr, de)

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

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

frontend/src/i18n/locales/fr.json

@@ -31,12 +31,12 @@
     "signIn": "Se connecter",
     "signOut": "Déconnexion",
     "signingIn": "Connexion en cours...",
-    "username": "Nom d'utilisateur",
+    "email": "E-mail",
     "password": "Mot de passe",
-    "enterUsername": "Entrez votre nom d'utilisateur",
+    "enterEmail": "Entrez votre e-mail",
     "enterPassword": "Entrez votre mot de passe",
     "welcomeBack": "Bon retour",
-    "pleaseEnterDetails": "Veuillez entrer vos informations pour vous connecter.",
+    "pleaseEnterDetails": "Veuillez entrer votre e-mail et mot de passe pour vous connecter.",
     "authError": "Erreur d'Authentification",
     "invalidCredentials": "Identifiants invalides",
     "orContinueWith": "Ou continuer avec",