Signet Testing

Signet is for testing, not production funds.

Recommended environment:

```bash

NEXT_PUBLIC_ENABLE_BITCOIN=true

BITCOIN_DEFAULT_NETWORK=signet

BITCOIN_EXPLORER_BASE_URL=https://mempool.space/signet

BITCOIN_RPC_URL=

BITCOIN_ALLOW_MOCK_UTXOS=false

```

Manual test flow:

1. Create wallet policy.

2. Copy receive address.

3. Fund with signet BTC.

4. Refresh UTXOs.

5. Build PSBT.

6. Export PSBT.

7. Sign externally.

8. Import signed PSBT.

9. Finalize.

10. Broadcast.

11. Verify on `mempool.space/signet`.