Support onboarding on mobile to a given HS + room by correlating sessions from web->mobile

From the internal matrix.to room:

> i see that Google and others have finally cracked the web->mobile onboarding flow challenge
> if you don’t have google photos installed and you click on a link, it prompts you to install the app
> and then it prompts you to copy an id to your clipboard
> which then it prompts you to paste in the app once installed
> so you can correlate the flows
> genius! and a really great UX
> we should do it :D

Start by reviewing the internal matrix.to discussion and the referenced screenshot, then identify which Element client repositories and onboarding entry points are involved. Define the web-to-mobile session correlation flow for a given homeserver and room, including how completion would be verified across clients.

Written by the indexing model from the issue text.