[M2] Integrate block explorer URL override for Otterscan

[augustocollerone]

Summary

This PR delivers part of Milestone 2: Frontend Integration of the CoW Grants Program RFP: CoW Protocol Playground Block Explorer Integration proposal by CoBuilders.

Updates the CoW Swap and Explorer Dockerfiles to use REACT_APP_BLOCK_EXPLORER_URL, enabling all block explorer links to point to the local Otterscan instance (port 8003) instead of Etherscan.

Milestone 2 Deliverables

CoW Swap Frontend Integration

• Configure REACT_APP_BLOCK_EXPLORER_URL=http://localhost:8003 in build
• Transaction hash links point to local Otterscan
• Address links point to local Otterscan
• All navigation flows work correctly

CoW Explorer Frontend Integration

• Configure REACT_APP_BLOCK_EXPLORER_URL=http://localhost:8003 in build
• "View on Blockchain" links point to local Otterscan
• Address and token links point to local Otterscan
• All navigation flows work correctly

Success Criteria

"One-click navigation from frontends to the local block explorer for all transactions and addresses"

• Clicking any explorer link in CoW Swap (port 8000) → Opens in Otterscan (port 8003)
• Clicking any explorer link in CoW Explorer (port 8001) → Opens in Otterscan (port 8003)

Changes

Modified Files

File	Changes
Dockerfile.cowswap	Added REACT_APP_BLOCK_EXPLORER_URL build arg
Dockerfile.explorer	Added REACT_APP_BLOCK_EXPLORER_URL env var

Architecture

┌─────────────┐     ┌─────────────┐
│  CoW Swap   │     │ CoW Explorer│
│ (port 8000) │     │ (port 8001) │
└──────┬──────┘     └──────┬──────┘
       │                   │
       │   All explorer    │
       │   links now       │
       │   point to ───────┤
       │                   │
       └───────────┬───────┘
                   │
                   ▼
           ┌─────────────┐
           │  Otterscan  │
           │ (port 8003) │
           └─────────────┘

Dependencies

This PR depends on:

• PR: cowprotocol/cowswap#XXX - Adds REACT_APP_BLOCK_EXPLORER_URL env var support

Testing

1. Start the playground:

   docker compose -f docker-compose.fork.yml up --build

2. Open CoW Swap at http://localhost:8000:

   • Make a swap or view transaction history
   • Click any "View on Explorer" or transaction link
   • Expected: Opens http://localhost:8003/tx/{hash}

3. Open CoW Explorer at http://localhost:8001:

   • Browse any order or transaction
   • Click any address or transaction link
   • Expected: Opens http://localhost:8003/address/{addr}

4. Verify Otterscan shows the transaction/address details with full traces

Demo

1. User clicks transaction in CoW Swap
2. Otterscan opens with full transaction details
3. Traces, logs, and gas profiling visible locally

Milestones

Milestone	Description	Status
M1	Otterscan Integration	PR #4000
M2	Frontend Integration	This PR (+ cowswap PR)
M3	Documentation	Pending

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Submitted by CoBuilders as part of the CoW Grants Program