fix(ethereum): default mainnet nodeURL to Alchemy's public endpoint

eth.llamarpc.com was unreachable — gateway logged 'Unable to fetch block
number' on every startup and all mainnet reads failed. eth-mainnet.g.alchemy.com/public
returns chainId 0x1.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: fengtality

src/templates/chains/ethereum/mainnet.yml

@@ -1,5 +1,5 @@
 chainID: 1
-nodeURL: https://eth.llamarpc.com
+nodeURL: https://eth-mainnet.g.alchemy.com/public
 nativeCurrencySymbol: ETH
 geckoId: eth
 transactionExecutionTimeoutMs: 10000 # Timeout for waiting for transaction execution (in milliseconds)