Change network indicator on the cover screen

[GBKS]

Please describe the feature you'd like to see added.

The cover screen currently shows a differently colored logo to indicate the network. Only people very closely vibing with Bitcoin Core will know what this means. Let's be more explicit, see the design.

Is your feature related to a problem, if so please describe it.

Here's what it currently looks like.

Describe the solution you'd like

See screenshot above.

Describe any alternatives you've considered

Not sure how much text we need to show. What speaks for being very explicit is that it reduces the risk of people being tricked by fraudsters. For example, someone pretends to do a fiat-bitcoin exchange, and the one person sends testnet bitcoin and the other person thinks they are real. Not sure if that's a real scenario, but we generally do want to be careful.

Main thing is that changing the logo color does not communicate a reason, or how to act accordingly.

Please leave any additional context

The small network indicator tag matches is the same we defined on the block clock screen (see design docs).

[GBKS]

We briefly discussed this in the jam session yesterday. One of the tricky things is what happens when users press the "Change network" button. On desktop, it sounds like they need to run the application with a different parameter. On mobile, it might not even be possible. So what do we tell people here?