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

Adds AudioOperations.js with pure argument builders (convert/trim/extract/merge)
plus one executeOperation that spawns ffmpeg via a dependency-injected execFileFn,
so tests never invoke a real binary. Wires process-audio-operation in main.js and
updates preload.js's ALLOWED_SEND_CHANNELS to replace the 5 stale audio-* entries.

Amit Haridas

src/main.js

@@ -6,6 +6,7 @@ const { execFile } = require('child_process');
 const WordTemplateExporter = require('./wordTemplateExporter');
 const PDFOperations = require('./main/PDFOperations');
 const ImageOperations = require('./main/ImageOperations');
+const AudioOperations = require('./main/AudioOperations');
 const GitOperations = require('./main/GitOperations');
 const PdfFontHeader = require('./main/PdfFontHeader');
 const MonospaceFontConfig = require('./main/MonospaceFontConfig');
@@ -4636,6 +4637,20 @@ ipcMain.handle('process-image-operation', async (event, { operation, data }) =>
   }
 });
 
+// ========================================
+// AUDIO OPERATIONS — delegates to main/AudioOperations.js
+// ========================================
+
+ipcMain.handle('process-audio-operation', async (event, { operation, data }) => {
+  try {
+    return await AudioOperations.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, {