Add notice that Element Desktop is running in the background

### Your use case

#### What would you like to do?

Add a notice for desktop users that when clicking the close button on the native app window, the application will still be running in the background.

#### Why would you like to do it?

[Discord received a notice by the CNIL to stop this](https://www.lemonde.fr/pixels/article/2022/11/17/la-cnil-inflige-a-discord-une-amende-de-800-000-euros_6150300_4408996.html) (article in French, sorry)

It is especially important to do in case of a VoIP call where camera and microphone will still be broadcasted if the app is running in the background

#### How would you like to achieve it?

I will leave that to competent folks to decide

### Have you considered any alternatives?

n/a

### Additional context

n/a

No files or tests are named. Start by locating the desktop app's native-window close handling and the existing notice or notification patterns; confirm how background execution is represented. Done means desktop users are clearly told that closing the window leaves the application running, including the VoIP privacy implication.

Written by the indexing model from the issue text.