[P2] ffmpeg discovery is Unix-only (which + hardcoded paths); duplicated in two modules

[eboudrant]

Priority: P2 · Review finding: #15

Both src/video.js and src/imageSlice.js carry their own (slightly diverged) findFfmpeg using which plus Homebrew//usr/bin paths. On Windows, which doesn't exist and the search paths are Unix-only, so ffmpeg is never found even when installed.

Fix: extract a shared src/ffmpeg.js; probe with where on win32 (or just spawnSync('ffmpeg', ['-version'])), and keep one search-path list.