Deliver EVM integration for governance portal

[lukozill]

Summary

Adds EVM wallet support (MetaMask, Ledger Live, Brave Wallet, Coinbase Wallet, and any
window.ethereum-injecting extension) as an alternative to the existing ZilPay connector in
the Governance Portal.

No new frontend dependencies — all EVM wallet interaction goes through the
browser-injected window.ethereum provider. @web3modal/standalone + @wagmi/core were
considered but rejected: wagmi v2 and viem are ESM-only and conflict with Vue CLI 4 /
Webpack 4, requiring invasive config changes not worth taking on in a legacy project. The
trade-off is that WalletConnect QR-code flow is not supported, which is acceptable because
the target users connect hardware Ledgers via MetaMask or Ledger Live.

Phase 1 — Decouple blockchain reads from ZilPay (governance-snapshot)

• Added src/helpers/zilliqa.ts — shared @zilliqa-js/zilliqa singleton pointed at
  https://api.zilliqa.com/
• Rewrote getBlockNumber and getTotalSupply in src/helpers/web3.ts to use the RPC
  singleton directly instead of the ZilPay provider object
• Rewrote the zrc2-balance-of strategy in src/helpers/get-scores.ts to use the RPC
  singleton
• Updated all callers in src/store/modules/app.ts and src/views/Create.vue — ZilPay is
  no longer involved in any read path

Phase 2 — EVM connector and store wiring (governance-snapshot)

• Added "evm" entry to src/helpers/connectors.json
• Added EVMConnector class to src/helpers/plugins/LockPlugin.ts — connects via
  eth_requestAccounts, session-checks via eth_accounts (no prompt), logout clears
  localStorage only
• Registered EVMConnector in src/auth.ts
• Updated loadProvider in src/store/modules/web3.ts: EVM path derives base16/bech32
  from the connected address using toBech32Address from @zilliqa-js/crypto; subscribes to
  accountsChanged events
• Updated signMessage in src/helpers/web3.ts: EVM path signs via personal_sign
  (EIP-191), returning { message, signature }; ZilPay Schnorr path unchanged
• Updated send() in src/store/modules/app.ts to include sigType: 'evm' | 'schnorr' in
  the API request body
• Fixed connector icon in src/components/Modal/Account.vue to show a generic wallet icon
  for non-ZilPay connectors

Phase 3 — Backend EVM signature verification (governance-api)

• Added ethers dependency
• Created lib/utils/verify-evm-signature.ts — verifies EIP-191 personal_sign signatures
  using ethers.utils.verifyMessage + ecrecover
• Updated lib/routes/message.ts: routes signature verification by sigType — 'evm' uses
  ECDSA recovery, default uses existing Schnorr path; all existing ZilPay and legacy
  submissions continue to work unchanged

Key design decisions

• EVM addresses are stored in the same format as ZilPay addresses (lowercase hex, no 0x
  prefix). The existing validation.isBech32 guard in message.ts already handles this
  correctly.
• All balances (including EVM wallet users') live on the Scilla side of Zilliqa, keyed by
  base16 address — no changes needed to balance retrieval or the 30 gZIL proposal minimum
  check.
• The Vuex web3 state shape ({ base16, bech32 }) is unchanged; bech32 is derived from
  the EVM address via toBech32Address.
• Session persistence follows the same _lock.connector localStorage pattern as ZilPay.

Test plan

• Locally ran the governance portal with governance docker compose
• Created a new vote in GZIL space
• Voted with ZilPay wallet (Schnorr path — regression)
• Voted with EVM wallet (MetaMask / window.ethereum — new path)