The Wallet page was Cashu-only and said 'App only' on web, so anyone with an
NWC wallet connected saw nothing. Adds:
- useNWC.getBalance() (millisats -> sats) and makeInvoice() via NWCClient
- NwcWalletCard: live balance + refresh, Receive (create invoice, QR, copy),
Send (paste invoice, decoded amount shown, pay via the connected wallet)
Renders only when an NWC connection is active.
