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/components/DevQuickLogin.tsx

@@ -5,7 +5,7 @@ import { useQueryClient } from '@tanstack/react-query';
 import { getBaseDomain, buildSubdomainUrl } from '../utils/domain';
 
 export interface TestUser {
-  username: string;
+  email: string;
   password: string;
   role: string;
   label: string;
@@ -14,56 +14,56 @@ export interface TestUser {
 
 const testUsers: TestUser[] = [
   {
-    username: 'superuser',
+    email: 'superuser@platform.com',
     password: 'test123',
     role: 'SUPERUSER',
     label: 'Platform Superuser',
     color: 'bg-purple-600 hover:bg-purple-700',
   },
   {
-    username: 'platform_manager',
+    email: 'manager@platform.com',
     password: 'test123',
     role: 'PLATFORM_MANAGER',
     label: 'Platform Manager',
     color: 'bg-blue-600 hover:bg-blue-700',
   },
   {
-    username: 'platform_sales',
+    email: 'sales@platform.com',
     password: 'test123',
     role: 'PLATFORM_SALES',
     label: 'Platform Sales',
     color: 'bg-green-600 hover:bg-green-700',
   },
   {
-    username: 'platform_support',
+    email: 'support@platform.com',
     password: 'test123',
     role: 'PLATFORM_SUPPORT',
     label: 'Platform Support',
     color: 'bg-yellow-600 hover:bg-yellow-700',
   },
   {
-    username: 'tenant_owner',
+    email: 'owner@demo.com',
     password: 'test123',
     role: 'TENANT_OWNER',
     label: 'Business Owner',
     color: 'bg-indigo-600 hover:bg-indigo-700',
   },
   {
-    username: 'tenant_manager',
+    email: 'manager@demo.com',
     password: 'test123',
     role: 'TENANT_MANAGER',
     label: 'Business Manager',
     color: 'bg-pink-600 hover:bg-pink-700',
   },
   {
-    username: 'tenant_staff',
+    email: 'staff@demo.com',
     password: 'test123',
     role: 'TENANT_STAFF',
     label: 'Staff Member',
     color: 'bg-teal-600 hover:bg-teal-700',
   },
   {
-    username: 'customer',
+    email: 'customer@demo.com',
     password: 'test123',
     role: 'CUSTOMER',
     label: 'Customer',
@@ -86,11 +86,11 @@ export function DevQuickLogin({ embedded = false }: DevQuickLoginProps) {
   }
 
   const handleQuickLogin = async (user: TestUser) => {
-    setLoading(user.username);
+    setLoading(user.email);
     try {
-      // Call token auth API
+      // Call token auth API - username field contains email since we use email as username
       const response = await apiClient.post('/auth-token/', {
-        username: user.username,
+        username: user.email,
         password: user.password,
       });
 
@@ -176,12 +176,12 @@ export function DevQuickLogin({ embedded = false }: DevQuickLoginProps) {
       <div className="grid grid-cols-2 gap-2">
         {testUsers.map((user) => (
           <button
-            key={user.username}
+            key={user.email}
             onClick={() => handleQuickLogin(user)}
             disabled={loading !== null}
             className={`${user.color} text-white px-3 py-2 rounded text-sm font-medium transition-colors disabled:opacity-50 disabled:cursor-not-allowed`}
           >
-            {loading === user.username ? (
+            {loading === user.email ? (
               <span className="flex items-center justify-center">
                 <svg className="animate-spin h-4 w-4 mr-2" viewBox="0 0 24 24">
                   <circle