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/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",