IAV-756/feat: integrate conversation history and client messages API endpoints

• Add paginated fetching for conversation threads with channel filtering and search
• Implement conversation detail view with paginated message loading on scroll
• Add debounced search for both history threads and client list
• Support infinite scroll for older messages in conversation detail
• Include channel filter dropdown (WhatsApp, Instagram, Messenger, Telegram)
• Wire up client list tab with search and detail view navigation

Closes IAV-756