fix: use btc utxos for node balance

[volcano303]

Summary

• use Bitcoin Core listunspent to calculate node-mode BTC address balance from current UTXOs
• keep the existing Esplora fallback when RPC UTXO lookup is unavailable
• add regression coverage for UTXO summing, spent-address zero balance, and fallback behavior

Issue

Closes #313

Validation

• uv run pytest tests/test_bitcoin_signing.py::TestBitcoinProviderGetBalance -q
• uv run pytest tests/test_bitcoin_signing.py -q
• uv run ruff check allways/ neurons/ tests/test_bitcoin_signing.py
• uv run pre-commit run --all-files
• uv run pytest tests/