Split tray UI into separate binary to remove CGO dependency

- Create new cmd/mcpproxy-tray binary for macOS system tray
- Add HTTP API endpoints in internal/api/handlers.go for inter-process communication
- Implement SSE (Server-Sent Events) for real-time status updates
- Create HTTPServerClient adapter implementing ServerInterface via HTTP calls
- Remove tray integration from core mcpproxy binary
- Delete tray_gui.go and tray_stub.go build-tagged files

This enables:
- CGO-free core binary for better cross-platform compatibility
- Independent tray binary that communicates via HTTP API
- Preserved functionality while eliminating build complexity
- Clean separation of concerns between core and UI

Core binary now builds with CGO_ENABLED=0 for full static linking.
Tray binary uses CGO for native macOS UI integration.

Author: Dumbris