Test coverage: MessagesApp.tsx (2,385 LOC) has zero tests

[jaylfc]

Context

MessagesApp.tsx is a 2,385-line monolithic component handling channels, messages, WebSocket, threads, pins, attachments, reactions, slash commands, typing indicators, and canvas viewing. It has zero direct test files.

Suggested Approach

1. Extract sub-components: MessageList, MessageInput, ChannelSidebar, ThreadPanel, ReactionBar
2. Write tests for each using Vitest + Testing Library
3. Integration test the WebSocket reconnection logic (mock WebSocket)
4. Test the custom markdown renderer for edge cases
5. The existing 15 test files in chat/__tests__/ are a good foundation