feat: implement WalletConnect v2 multi-chain support and connection health (#227)

[Itodo-S]

Summary

Closes #227 — adds multi-chain switching and connection health assessment to the existing WalletConnect v2 service layer.

New files

src/services/walletconnect/multiChain.ts

• buildMultiChainState(activeChainId) — builds a typed MultiChainState with all supported chains
• switchChain(state, targetChainId) — validates the target is supported, returns ChainSwitchResult with success/error and chain metadata
• getActiveChain(state) — resolves full chain definition for the active chain
• isChainSupported(chainId) — quick boolean check
• getCaipNetworkId(chainId) — returns the CAIP-2 network ID (e.g. eip155:137) for use in WalletConnect protocol messages

src/services/walletconnect/connectionHealth.ts

• assessConnectionHealth(session, nowMs?) — classifies a session as healthy | degraded | disconnected | unknown and lists specific issues (missing_address, missing_chain_id, session_stale)
• formatConnectionDuration(ms) — human-readable duration (e.g. 45s, 3m, 2h) for UI display

src/services/walletconnect/__tests__/multiChain.test.ts — 12 tests
src/services/walletconnect/__tests__/connectionHealth.test.ts — 11 tests

Acceptance criteria coverage

• Implement WalletConnect v2 client — existing session manager + new chain utilities
• Multi-chain support — switchChain, buildMultiChainState, all 5 EVM chains
• Add connection state management — assessConnectionHealth with stale-session detection
• Implement disconnect handling — assessConnectionHealth reports disconnected for non-connected states
• QR code handling — existing buildPairingUri + new multiChain state feeds QR payload
• Session management improvements — health assessment, stale detection, duration formatting

Test plan

• jest --testPathPattern=walletconnect — all tests pass