Auto-detect network from TXID

[4rkad]

When a user submits a TXID, automatically detect which network it belongs to by querying multiple backends:

• Mainnet (mempool.space/api/tx/{txid})
• Testnet3 (mempool.space/testnet/api/tx/{txid})
• Testnet4 (mempool.space/testnet4/api/tx/{txid})
• Signet (mempool.space/signet/api/tx/{txid})

The first API that returns a valid response identifies the network. This avoids the user having to manually select the network or getting confusing errors when analyzing a tx from the wrong network.