Add experimental Windows desktop support

[sjmiller609]

summary

• add a Windows desktop manager using native cursor, mouse, keyboard, and clipboard APIs
• add a Windows capture backend that reads VP8 desktop frames from FFmpeg gdigrab
• retain the existing Neko HTTP, WebSocket, WebRTC, and client contracts
• keep the existing X11/GStreamer implementation unchanged behind non-Windows build constraints

validation

• cross-compiled with GOOS=windows GOARCH=amd64 CGO_ENABLED=0 go build ./cmd/neko
• ran Neko inside a Windows 11 QEMU guest
• established 1280x800 WebRTC video and audio tracks
• navigated Chrome through Neko mouse and keyboard input
• restarted the guest and verified the interactive desktop and Neko recovered without an external desktop client

current limitations

• audio emits valid Opus silence rather than capturing the Windows output device
• screen resizing, touch input, file drop, file chooser integration, and screenshot capture are not implemented
• FFmpeg must be available as ffmpeg.exe on PATH
• this is an experimental first pass, not production-ready Windows parity