fix(signer): sanitize chain state on reconnect to prevent stale address

[mehmetkr-31]

Summary

Fixes #214 — incorrect or missing public address after reconnecting signer.

Root Cause

When Signer is reconstructed after cleanup() (e.g., page reload + reconnect):

1. The constructor reads account.chain from the store.
2. If the store was cleared by cleanup(), account.chain is undefined.
3. The fallback params.metadata.appChainIds?.[0] ?? 1 could be stale, missing, or default to chain id 1 (Ethereum mainnet) even when the user was on Base (8453).
4. This caused this.accounts to remain empty until the next handshake, and handleGetCapabilitiesRequest to throw unauthorized.

Fix

Prefer the chains array already registered in the store before falling back to constructor metadata:

Constructor:

this.chain = account.chain ?? chains?.[0] ?? {
  id: params.metadata.appChainIds?.[0] ?? 1,
};

cleanup():

const chains = this.storeInstance.getState().chains;
this.chain = chains?.[0] ?? {
  id: metadata?.appChainIds?.[0] ?? 1,
};

This ensures the correct chain context (e.g., Base mainnet 8453) is preserved across reconnect cycles instead of silently falling back to Ethereum mainnet.

Test Plan

• yarn workspace @base-org/account test --run → 999/999 pass

Related Issues

• Closes Bug: Account SDK returns incorrect or missing public address after reconnecting signer #214

[cb-heimdall]

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