Require an explicit red-box selection before extracting UI-design assets: add rectangular/ellipse/brush selection UI, render red-outline marks into a marked Data URL on the client, and update the extraction prompt to "仅提取被红色框框选的素材并整理成spritesheet". Add frontend overlay/raster models and tests to support the selection flow and adjust numerous image-editor tests and views to wire up the selection lifecycle. On the dev ops side, enhance scripts/dev to detect a local %LOCALAPPDATA%/Genarrative/ffmpeg/bin installation on Windows, prepend it to PATH and inject CHARACTER_ANIMATION_FFMPEG_PATH/FFPROBE_PATH (with tests), and export the helper. Server config now prefers GENARRATIVE_*-prefixed FFmpeg env vars and includes a Rust test to validate reading the prefixed variables. Update docs to describe the new UI extraction behavior and local FFmpeg handling.