Design suggestions for mobile guide toast

[jryans]

In matrix-org/matrix-react-sdk#5586, we added an optional mobile guide toast to nudge people towards the native apps. It's disabled by default.

@niquewoodhouse provided some good feedback we may want to address in the future:

- When does this appear? Is it noticeable enough?

If people's experience of Element will be broken, they'll be more likely to decide not to return/think of the product as useful. So, if we have a modal component, I'd suggest it might be better to block people briefly, but at least be sure they know we have stable options somewhere?

Basic example

One thing to keep in mind is that we're expecting some users to complete a 3PID room invite flow and after that head to a mobile app, so we wouldn't necessarily want to overlay the whole screen when you first land. Ideally, we can revisit this invite flow holistically, as it's all a bit of a mess at the moment.

[MadLittleMods]

cc @daniellekirkwood since you recently did some Figma designs around the flows explained in matrix-org/matrix.to#263

[niquewoodhouse]

I think a custom toast/banner could work fine but I think using smart app banners is more familiar to users, so going to close this suggestion in favour of that.