Chatwoot-style 3-panel chat interface:
- Left panel: conversation list with filters (status, assignee, search)
- Center panel: message thread with reply box and AI draft
- Right panel: contact details, assignment, internal notes
Features:
- Real-time thread loading via fetch API
- Keyboard shortcuts (Cmd+Enter to send)
- Collision detection heartbeat
- New conversation modal
- Thread status management (close/reopen)
- AI reply generation
- Internal notes
Routes added at /s/{business}/chat/*
Sidebar link added under Inbox section (requires Sales Suite)
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