Build a Local Depth Video Converter with Gradio

Build a complete Python Depth video converter that runs locally on Windows and macOS, without relying on any existing project code. 1. Use Gradio for a simple web UI and support MP4 / MOV uploads. 2. Use Depth Anything V2 to convert each frame into a grayscale Depth video. 3. Automatically use NVIDIA CUDA on Windows when available, Apple Silicon acceleration on Mac when available, otherwise fall back to CPU. 4. Include model size selection, output resolution, black/white inversion, temporal smoothing to reduce flicker, optional original audio preservation with ffmpeg, and export to a compatible MP4. Also generate requirements.txt, README.md, a complete runnable Python script, and clear installation and launch commands for both Windows and macOS.