feat: detect dropped and replaced transactions

[AugustoL]

Summary

When a user submits a transaction with the same nonce as a pending transaction but with higher gas (to speed it up or cancel it), the original transaction gets dropped from the mempool and replaced. Etherscan detects and surfaces this — we want to do the same in OpenScan.

Motivation

Users often replace stuck transactions and then can't find the original in the explorer. Surfacing the "dropped & replaced" status gives users clarity on what happened to their transaction.

Proposed Solution

Research how to detect dropped/replaced transactions and display their status in the transaction detail page. The likely approach:

1. Detection: when a transaction hash returns null from eth_getTransactionByHash (not found), check if a transaction with the same nonce from the same sender was mined — if so, the original was dropped and replaced by it.
2. Display: show a "Dropped & Replaced" status badge on the transaction page, with a link to the replacement transaction.

Research Questions

• Does eth_getTransactionByHash return null for dropped txs, or do some nodes keep them?
• Can we reliably detect the replacement tx by scanning nonce + sender in recent blocks?
• Should we query the mempool (txpool_content) to check if the tx is still pending?
• How does Etherscan's API expose this? (status field, separate endpoint?)
• Are there RPC providers that expose a dedicated "dropped tx" endpoint?
• Performance implications: how many blocks back do we need to scan to find the replacement?

Acceptance Criteria

• Research complete — detection approach documented
• Transaction page shows "Dropped & Replaced" status when applicable
• Link to the replacement transaction is shown
• Graceful fallback when replacement cannot be determined (e.g. show "Dropped" without replacement link)
• Works on Ethereum mainnet; best-effort on L2s