feat: add connected accounts callback handling

[yogeshchoudhary147]

Description

Adds callback handling for connected accounts, completing the implementation started in #767.

Previously, the SDK could initiate the connect flow with connectAccountWithRedirect() but couldn't process the response when users returned from connecting their account. This PR adds the missing callback logic.

Changes

• Update AppState interface with connectedAccount and response_type fields
• Export ConnectedAccount, ResponseType, ConnectError, and ConnectAccountRedirectResult types
• Detect connect_code parameter in callback URL (in addition to code)
• Extract connected account data from redirect result and add to appState$
• Add tests for callback handling (4 new tests, 131 total passing)
• Update documentation showing how to access connected account data

Usage

After connecting an account, access the details via appState$:

this.auth.appState$.subscribe((appState) => {
  if (appState.connectedAccount) {
    console.log('Connected:', appState.connectedAccount.connection);
  }
});

Testing

• ✅ All tests pass (131 total)
• ✅ Production build successful
• ✅ Types exported correctly