docs: add Arc network chain ID reference table to ACCOUNTS.md

[SAMUNCLE155]

Summary

Added a consolidated reference table of Arc network chain IDs to ACCOUNTS.md to help developers quickly identify the correct chain ID for each network.

The table includes Local Dev (1337), Devnet (5042001), Testnet (5042002), and Mainnet (5042), along with their purposes and source locations.

Why

• ACCOUNTS.md already documents local dev accounts at chain ID 1337, but does not list the other Arc networks
• Having all chain IDs in one place reduces confusion and prevents developers from using incorrect chain IDs (e.g., 1516 mentioned in some external documentation is not a valid Arc chain ID)
• The chain IDs are sourced from the genesis config files at assets/{network}/genesis.config.ts

Changes

• Added a ## 🔗 Arc Network Chain IDs section to ACCOUNTS.md
• Listed all four Arc networks with their chain IDs and purposes

Checklist

• Documentation only — no functional code changes
• Chain IDs verified against genesis config files in the repo