add ReceivePage UI for the demo wallet

[Johnosezele]

Adds the bdk_demo Receive screen on top of the already merged receive address service/provider work (PR #79). Users can generate a new persisted receive address, view it as a QR code, copy it to the clipboard, and see the derivation index.

Notes to reviewers

• This PR is UI + routing + presentation tests only; it does not change WalletService.generateAddress() or currentReceiveAddressProvider.
• Receive remains offline reachable (unlike /send, which redirects when offline).
• If generating a new address fails after a prior success, the UI keeps the previous valid address visible and shows the error beneath it.

[reez]

ACK 4321f15