Replace WebSocket-based real-time communication with Server-Sent Events (SSE) across the messaging infrastructure. This includes:
- Create new SSEClient datasource to manage SSE connections
- Create new SSEMessageHandler to process SSE events
- Update ProcessMessageUseCase to use SSEClient instead of WebSocketClient
- Update MessageManager and MessageStateManager to work with SSE handlers
- Rename connection state variables from `isWebSocketConnected` to `isSSEConnected`
- Update type definitions in dto.ts (WSEventType → SSEEventType, etc.)
- Delete obsolete WebSocketClient and WebSocketMessageHandler files
- Update comments and documentation to reflect SSE terminology
This refactoring aligns with the backend's SSE implementation for better compatibility with React Native's networking capabilities.
