chore: improve etherscan v2 DX (configurable chainid + explicit errors)

[sanbir]

Summary

Follow-up to the Etherscan V2 migration for Contract Viewer.

Problem

After switching from the deprecated V1 endpoint to V2, there are two developer-experience gaps:

1. chainid is hardcoded to mainnet (1), making non-mainnet usage harder.
2. parser errors collapse to a generic etherscan response is not OK, which hides actionable upstream messages (e.g. invalid API key, unverified source, rate limits).

Proposal

• Make chain id configurable via env (e.g. ETHERSCAN_CHAIN_ID, default 1).
• Preserve Etherscan result/message in thrown parser errors so users can diagnose quickly.

Context

• Related PR: fix: bump etherscan api to v2 #398
• Affected paths: util/EtherscanApi.ts, util/EtherscanParser.ts