fix: stage libffmpeg for Linux AppImage child processes

scripts/electron-builder-after-pack.cjs

@@ -0,0 +1,41 @@
+const fs = require('node:fs/promises');
+const path = require('node:path');
+
+const LINUX_FFMPEG_LIBRARY = 'libffmpeg.so';
+
+async function stageLinuxAppImageSharedLibrary(
+  context,
+  deps = {
+    access: (filePath) => fs.access(filePath),
+    mkdir: (dirPath) => fs.mkdir(dirPath, { recursive: true }),
+    copyFile: (from, to) => fs.copyFile(from, to),
+  },
+) {
+  if (context.electronPlatformName !== 'linux') return false;
+
+  const sourceLibraryPath = path.join(context.appOutDir, LINUX_FFMPEG_LIBRARY);
+
+  try {
+    await deps.access(sourceLibraryPath);
+  } catch {
+    return false;
+  }
+
+  const targetLibraryDir = path.join(context.appOutDir, 'usr', 'lib');
+  const targetLibraryPath = path.join(targetLibraryDir, LINUX_FFMPEG_LIBRARY);
+
+  await deps.mkdir(targetLibraryDir);
+  await deps.copyFile(sourceLibraryPath, targetLibraryPath);
+
+  return true;
+}
+
+async function afterPack(context) {
+  await stageLinuxAppImageSharedLibrary(context);
+}
+
+module.exports = {
+  LINUX_FFMPEG_LIBRARY,
+  stageLinuxAppImageSharedLibrary,
+  default: afterPack,
+};