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/de.json

@@ -31,12 +31,12 @@
     "signIn": "Anmelden",
     "signOut": "Abmelden",
     "signingIn": "Anmeldung läuft...",
-    "username": "Benutzername",
+    "email": "E-Mail",
     "password": "Passwort",
-    "enterUsername": "Geben Sie Ihren Benutzernamen ein",
+    "enterEmail": "Geben Sie Ihre E-Mail-Adresse ein",
     "enterPassword": "Geben Sie Ihr Passwort ein",
     "welcomeBack": "Willkommen zurück",
-    "pleaseEnterDetails": "Bitte geben Sie Ihre Daten ein, um sich anzumelden.",
+    "pleaseEnterDetails": "Bitte geben Sie Ihre E-Mail-Adresse und Ihr Passwort ein, um sich anzumelden.",
     "authError": "Authentifizierungsfehler",
     "invalidCredentials": "Ungültige Anmeldedaten",
     "orContinueWith": "Oder fortfahren mit",

frontend/src/i18n/locales/en.json

@@ -56,12 +56,12 @@
     "signIn": "Sign in",
     "signOut": "Sign Out",
     "signingIn": "Signing in...",
-    "username": "Username",
+    "email": "Email",
     "password": "Password",
-    "enterUsername": "Enter your username",
+    "enterEmail": "Enter your email",
     "enterPassword": "Enter your password",
     "welcomeBack": "Welcome back",
-    "pleaseEnterDetails": "Please enter your details to sign in.",
+    "pleaseEnterDetails": "Please enter your email and password to sign in.",
     "authError": "Authentication Error",
     "invalidCredentials": "Invalid credentials",
     "orContinueWith": "Or continue with",

frontend/src/i18n/locales/es.json

@@ -31,12 +31,12 @@
     "signIn": "Iniciar sesión",
     "signOut": "Cerrar Sesión",
     "signingIn": "Iniciando sesión...",
-    "username": "Nombre de usuario",
+    "email": "Correo electrónico",
     "password": "Contraseña",
-    "enterUsername": "Ingresa tu nombre de usuario",
+    "enterEmail": "Ingresa tu correo electrónico",
     "enterPassword": "Ingresa tu contraseña",
     "welcomeBack": "Bienvenido de nuevo",
-    "pleaseEnterDetails": "Por favor ingresa tus datos para iniciar sesión.",
+    "pleaseEnterDetails": "Por favor ingresa tu correo electrónico y contraseña para iniciar sesión.",
     "authError": "Error de Autenticación",
     "invalidCredentials": "Credenciales inválidas",
     "orContinueWith": "O continuar con",

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",