Add staff email client with WebSocket real-time updates

Implements a complete email client for platform staff members:

Backend:
- Add routing_mode field to PlatformEmailAddress (PLATFORM/STAFF)
- Create staff_email app with models for folders, emails, attachments, labels
- IMAP service for fetching emails with folder mapping
- SMTP service for sending emails with attachment support
- Celery tasks for periodic sync and full sync operations
- WebSocket consumer for real-time notifications
- Comprehensive API viewsets with filtering and actions

Frontend:
- Thunderbird-style three-pane email interface
- Multi-account support with drag-and-drop ordering
- Email composer with rich text editor
- Email viewer with thread support
- Real-time WebSocket updates for new emails and sync status
- 94 unit tests covering models, serializers, views, services, and consumers

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -65,6 +65,7 @@ const PlatformUsers = React.lazy(() => import('./pages/platform/PlatformUsers'))
 const PlatformStaff = React.lazy(() => import('./pages/platform/PlatformStaff'));
 const PlatformSettings = React.lazy(() => import('./pages/platform/PlatformSettings'));
 const BillingManagement = React.lazy(() => import('./pages/platform/BillingManagement'));
+const PlatformStaffEmail = React.lazy(() => import('./pages/platform/PlatformStaffEmail'));
 const ProfileSettings = React.lazy(() => import('./pages/ProfileSettings'));
 const VerifyEmail = React.lazy(() => import('./pages/VerifyEmail'));
 const EmailVerificationRequired = React.lazy(() => import('./pages/EmailVerificationRequired'));
@@ -585,6 +586,7 @@ const AppContent: React.FC = () => {
             )}
             <Route path="/platform/support" element={<PlatformSupportPage />} />
             <Route path="/platform/email-addresses" element={<PlatformEmailAddresses />} />
+            <Route path="/platform/email" element={<PlatformStaffEmail />} />
             <Route path="/help/guide" element={<HelpGuide />} />
             <Route path="/help/ticketing" element={<HelpTicketing />} />
             <Route path="/help/api" element={<HelpApiDocs />} />