[Feature]: "Keep app open" reminder when viewing LN invoice

[catch-21]

Describe the problem

Bitkit needs to remain in the foreground to receive payment via LN. We know some users will produce an invoice and copy it to send from another app on the same device, this will often not work.

See internal discussion https://synonymworkspace.slack.com/archives/C01H106JRQB/p1723735706112959

Describe the solution

We could do with a reminder on the LN invoice page so the user is aware of this limitation.

Additional context

Include in automated test suite.