Add src/main/VideoOperations.js with pure argument-builder functions
(buildConvertArgs, buildCompressArgs, buildTrimArgs, buildFramesArgs,
buildGifArgs) and a single executeOperation entry point that spawns
ffmpeg via dependency-injected execFileFn, mirroring AudioOperations.js.
Wire ipcMain.handle('process-video-operation', ...) in main.js using
getFFmpegPath() and sanitizeErrorMessage(). Update preload.js's
ALLOWED_SEND_CHANNELS: remove 6 stale video-* channel names, add
process-video-operation.
Amit Haridas
