Remove use of __dirname from main process

ts/util/appRootDir.main.ts

@@ -0,0 +1,14 @@
+// Copyright 2026 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { app } from 'electron';
+import { join } from 'node:path';
+
+export function getAppRootDir(): string {
+  // We have this logic because test-node runs under `electron-mocha` that has
+  // `app.getAppPath()` pointing within `electron-mocha`'s folder.
+  if (app.isPackaged || process.env.IS_BUNDLED) {
+    return app.getAppPath();
+  }
+  return join(__dirname, '..', '..');
+}