feat(video): implement ffmpeg-based video operations backend

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

src/main.js

@@ -7,6 +7,7 @@ const WordTemplateExporter = require('./wordTemplateExporter');
 const PDFOperations = require('./main/PDFOperations');
 const ImageOperations = require('./main/ImageOperations');
 const AudioOperations = require('./main/AudioOperations');
+const VideoOperations = require('./main/VideoOperations');
 const GitOperations = require('./main/GitOperations');
 const PdfFontHeader = require('./main/PdfFontHeader');
 const MonospaceFontConfig = require('./main/MonospaceFontConfig');
@@ -4651,6 +4652,20 @@ ipcMain.handle('process-audio-operation', async (event, { operation, data }) =>
   }
 });
 
+// ========================================
+// VIDEO OPERATIONS — delegates to main/VideoOperations.js
+// ========================================
+
+ipcMain.handle('process-video-operation', async (event, { operation, data }) => {
+  try {
+    return await VideoOperations.executeOperation(operation, data, {
+      ffmpegPath: getFFmpegPath(),
+    });
+  } catch (error) {
+    return { success: false, error: sanitizeErrorMessage(error.message) };
+  }
+});
+
 // IPC Handler for folder selection (for batch image operations)
 ipcMain.on('select-image-folder', (event, inputId) => {
   const folder = dialog.showOpenDialogSync(mainWindow, {