bdk_demo: Expose current receive address state in the demo wallet

[Johnosezele]

Add Riverpod state for the active wallet’s latest generated receive address so future receive UI can call one provider method instead of reaching into WalletService directly.

Part of #5

[j-kon]

Completed by #79, which added currentReceiveAddressProvider and the active-wallet receive-address state described here. The remaining Receive page, QR code, copy action, and UI tests are tracked separately in #83.