feat: Add ticket permission UI and fix assignee dropdown

Assignee Dropdown:
- Fix useUsers hook to fetch from /api/staff/ endpoint
- Add useStaffForAssignment hook for formatted dropdown data
- Update TicketModal to use new hook for assignee selection

Staff Permissions UI:
- Add "Can access support tickets" permission to invite modal for both managers and staff
- Add permission to edit modal for both managers and staff
- Managers default to having ticket access enabled
- Staff default to having ticket access disabled (must be explicitly granted)

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

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

frontend/src/components/TicketModal.tsx

@@ -3,7 +3,7 @@ import { useTranslation } from 'react-i18next';
 import { X, User, Send, MessageSquare, Clock, AlertCircle } from 'lucide-react';
 import { Ticket, TicketComment, TicketStatus, TicketPriority, TicketCategory, TicketType } from '../types';
 import { useCreateTicket, useUpdateTicket, useTicketComments, useCreateTicketComment } from '../hooks/useTickets';
-import { useUsers } from '../hooks/useUsers'; // Assuming a useUsers hook exists to fetch users for assignee dropdown
+import { useStaffForAssignment } from '../hooks/useUsers';
 import { useQueryClient } from '@tanstack/react-query';
 
 interface TicketModalProps {
@@ -34,7 +34,7 @@ const TicketModal: React.FC<TicketModalProps> = ({ ticket, onClose, defaultTicke
   const [isInternalComment, setIsInternalComment] = useState(false);
 
   // Fetch users for assignee dropdown
-  const { data: users = [] } = useUsers(); // Assuming useUsers fetches all relevant users (staff/platform admins)
+  const { data: users = [] } = useStaffForAssignment();
 
   // Fetch comments for the ticket if in detail/edit mode
   const { data: comments, isLoading: isLoadingComments } = useTicketComments(ticket?.id);