Implement reliable Windows system-audio loopback capture

[rancy777]

Goal

Replace or harden the current ffmpeg device-name-based system-audio capture so Windows loopback recording works reliably.

Current state

SystemAudioCaptureService depends on an input device exposed to ffmpeg. Many machines do not expose Stereo Mix or an equivalent input.

Useful direction

Investigate WASAPI loopback capture, define the mixing/synchronization strategy for microphone + system audio, and document supported Windows versions.

Acceptance criteria

• System audio can be captured on a normal Windows 10/11 machine without enabling Stereo Mix.
• Microphone-only, system-only, and combined modes are tested.
• Failure states are visible and do not lose the screen recording.
• Test hardware, Windows version, and verification steps are documented.