feat: multi-wallet architecture with localStorage state, EIP-712 verification, cross-tab WS leadership

- wallets.js: WalletManager state management with chain validation
- verifier.js: WalletVerifier EIP-712 signing via window.ethereum
- stream.js: WalletStreamManager with BroadcastChannel leader election, backoff + jitter
- AGENTS.md: updated with coding guidelines

test/test_setup.mjs

@@ -0,0 +1,4 @@
+globalThis.location = {
+  protocol: 'http:',
+  host: 'localhost:8080'
+};