Fix cross-tab race condition in silent SIWE refresh-token rotation

[NkechiOgbuji]

Close #305

Description

What changed

• Added a cross-tab refresh coordination layer to ensure only one browser tab performs a silent SIWE refresh at a time.
• Preserved the existing single-tab behavior while gracefully falling back when cross-tab coordination APIs are unavailable.
• Added timeout handling so waiting tabs can recover if the coordinating tab exits unexpectedly.
• Updated the refresh token contract documentation to describe the new coordination mechanism.
• Added unit tests for the coordination logic and extended the SIWE flow tests to verify only a single refresh request is issued during concurrent refresh attempts.

Verification

• npm test
• npm run typecheck
• npm run lint