[5 BOUNTY] [TypeScript] Add single-flight token refresh handling

[alKanz17]

What this does

Replaces the racy token refresh logic with a single-flight coordination pattern, eliminating the multi-tab refresh race.

How it works

• singleFlightRefresh() — stores the in-flight HTTP call in a module-level promise. All concurrent callers (same tab or cross-tab) share the same promise, guaranteeing exactly one /auth/refresh request.
• BroadcastChannel API — after a successful refresh, the winning tab broadcasts the new tokens via a BroadcastChannel. Sibling tabs receive the message and update their local state without making redundant refresh calls. The TOKEN_UPDATE_EVENT carries the full AuthTokens object.
• listenForTokenUpdates() — registers a BroadcastChannel message handler that validates and applies incoming tokens, then reschedules the local refresh timer.
• Graceful fallback — BroadcastChannel API availability is checked lazily. When unavailable (Node, older browsers), the single-flight still works within the same tab and cross-tab coordination is silently skipped.
• Cleanup — refreshPromise is nulled in the finally block, and stale tokens are cleared on failure.

Closes #1