feat: add WalletConnect connector support (#298)

[waterWang]

Summary

Adds WalletConnect v2 connector support to the wagmi wallet configuration, allowing mobile wallet users and hardware wallet users to connect via WalletConnect QR code.

Changes

Validation layer

• lib/wallet/validation.js: Added 'walletConnect' to SUPPORTED_CONNECTOR_NAMES
• lib/wallet/validation.d.ts: Updated type declaration to include 'walletConnect' in the readonly tuple

Connector layer

• lib/wallet/connectors.ts: Removed corrupted local declaration that conflicted with re-export from validation.js. The file now cleanly re-exports from validation.js with TypeScript type resolution via validation.d.ts
• lib/wallet/config.ts:
  • Added missing splitCsv import (pre-existing bug that caused ReferenceError at runtime)
  • Replaced fail() call with throw new ConfigError() in the walletConnect case (the fail helper was not imported in this module's scope)

Tests

• test/wallet-connectors.test.ts: Added tests for walletConnect parsing (standalone and combined with injected)
• test/wallet-config.test.ts: Added tests for WalletConnect config building (with project ID, combined connectors, and missing project ID error path). Added NEXT_PUBLIC_WALLETCONNECT_PROJECT_ID to the env cleanup list to prevent cross-test leakage

Acceptance Criteria ✓

• NEXT_PUBLIC_WALLET_CONNECTORS=walletConnect (alone or combined with injected) produces a working connector without throwing
• Omitting NEXT_PUBLIC_WALLETCONNECT_PROJECT_ID while walletConnect is enabled throws a clear ConfigError at startup
• .env.example and README document the new connector and variable (already documented)
• Tests cover the new connector
• All existing tests pass (40/40 tests passing)

Closes #298