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/i18n/locales/en.json

@@ -4041,5 +4041,49 @@
       "description": "Can't find what you're looking for? Our support team is ready to help.",
       "contactSupport": "Contact Support"
     }
+  },
+  "staffEmail": {
+    "title": "Staff Email",
+    "compose": "Compose",
+    "inbox": "Inbox",
+    "sent": "Sent",
+    "drafts": "Drafts",
+    "trash": "Trash",
+    "archive": "Archive",
+    "spam": "Spam",
+    "folders": "Folders",
+    "labels": "Labels",
+    "noEmails": "No emails",
+    "selectEmail": "Select an email to read",
+    "searchPlaceholder": "Search emails...",
+    "reply": "Reply",
+    "replyAll": "Reply All",
+    "forward": "Forward",
+    "markAsRead": "Mark as read",
+    "markAsUnread": "Mark as unread",
+    "star": "Star",
+    "unstar": "Unstar",
+    "moveToTrash": "Move to trash",
+    "delete": "Delete",
+    "restore": "Restore",
+    "send": "Send",
+    "saveDraft": "Save draft",
+    "discard": "Discard",
+    "to": "To",
+    "cc": "Cc",
+    "bcc": "Bcc",
+    "subject": "Subject",
+    "from": "From",
+    "date": "Date",
+    "attachments": "Attachments",
+    "clickToReply": "Click here to reply...",
+    "newMessage": "New Message",
+    "emailSent": "Email sent",
+    "draftSaved": "Draft saved",
+    "emailArchived": "Email archived",
+    "emailTrashed": "Email moved to trash",
+    "emailRestored": "Email restored",
+    "syncComplete": "Emails synced",
+    "syncFailed": "Failed to sync emails"
   }
 }