Although it's not quite the same as a native app, we already have built in support for desktop notifications on all browsers that support them, including some older WebKits.
There are still a great many organizations that are using XP. It's a reliable and battle tested OS. Ignoring these users wouldn't be a good idea, I don't think.
you can build it on .NET. Don't care about XP, FUCK THESE XP USERS.