Set AUMID to appId

It is also important to set the Application User Model ID (AUMID) to the
appId of the application, in order for notifications on Windows 8/8.1 to
function and for Window 10 notifications to display the app icon within
the notifications by default. The AUIMD should be set within the Main
process and before any BrowserWindows have been opened, it is normally
the first piece of code executed.

https://github.com/electron-userland/electron-builder/wiki/NSIS#guid-vs-application-name

// FREEBIE

main.js

@@ -9,6 +9,8 @@ const url = require('url')
 const autoUpdater = require('electron-updater').autoUpdater
 const autoUpdaterInterval = 60 * 60 * 1000;
 
+app.setAppUserModelId('org.whispersystems.signal-desktop')
+
 // Keep a global reference of the window object, if you don't, the window will
 // be closed automatically when the JavaScript object is garbage collected.
 let mainWindow