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/api/auth.ts

@@ -5,7 +5,7 @@
 import apiClient from './client';
 
 export interface LoginCredentials {
-  username: string;
+  email: string;
   password: string;
 }