Welcome is well used on its translation to Spanish, but I prefer it as -Bienvenido because it's efficient in marketing. Telegram Desktop is the register name of the app, so in Spanish it has to maintained its name. The diminutive -app is valid for both languages. I think that using both words app and official will be important for the client because it confers the certainty of using a trusting program. The other translations are the most similar into Spanish and I consider them correct.

Shorter, and simple

I use the @ because in spanish you have to specify the gender in the words

In Spanish, "bienvenido" is referred to a male, while "bienvenida" is referred to a female. A way to apply the word to both, males and females, is using "Bienvenido(a)" instead. People use "bienvenido" if they are forced to choose.