Add RDPService with session lifecycle management (connect, disconnect,
input forwarding, frame retrieval) using an injected backend factory.
Implement MockBackend that generates animated test frames with a
blue-purple gradient, bouncing color-cycling rectangle, pulsing circle,
and grid overlay for canvas renderer verification on macOS without
FreeRDP. The real FreeRDP purego bindings are deferred to Windows.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
