feat: enable supported Blockscout features

[JOY (JOY)]

Summary

• enable supported shared Blockscout backend features for Mainnet and Testnet
• add the Testnet User Ops Indexer and deployment runtime guards
• keep RPC-dependent pending and internal transaction fetchers disabled only on Testnet
• update the Vietnamese dark-theme feature inventory

Validation

• independent reviewer approved after rollback safety fixes
• Blockscout environment parity validation
• Python compile
• Docker Compose validation for Mainnet and Testnet
• HTML validation and browser render with no console warnings or errors
• git diff --check

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