Provide Link/QR-Code for Andoid/iOS App Installation & Setup

### Your use case

#### What would you like to do?

Upon first login onto the web app, prompt the user to setup Element on Android/iOS with minimal effort.

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

When signing up new students and staff, at first they often login only onto the web app in their browser. They often forget to open the web app in their browser after a few days and become unreachable.

By giving instructions to effortlessly setup Element on Android/iOS upon first login into the web app, they remain reachable and they will see more benefit in permanently setting up Element on there desktop/laptop computer.

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

Upon first login show a prompt to install Element on Android/iOS. As there is much available screen space with a newly setup Matrix account, this could be shown e.g. in the lower left corner or somewhere in the center.

This prompt should show a QR code which enables effortless setup, by requiring the user to only scan the QR code once for a complete Element setup without requiring the user to input information such as the home server address or username.

Upon scanning the QR code with the phone once, the phone should automatically do the following:
- Prompt the user to install the Element Android/iOS app.
- Show the user the Element Android/iOS app login screen, prefilled with the correct homeserver address and username.
- (if possible) Perform automatic device verification without user interaction.

It may be necessary to select the preffered app store within Element web, before showing the QR-code to the user.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_