fix: defer initial token refresh until first subscriber

[gjtorikian]

Summary

• The client TokenStore singleton is constructed at module-evaluation time, before hydration. When the middleware-set fast cookie held an already-expiring token, the constructor scheduled a 0ms refresh timer that could fire before Next.js initialized its router action queue.
• When that happens, the Server Action dispatch throws Internal Next.js error: Router action dispatched before initialization (E668) inside React 19's startTransition, which routes the error to reportGlobalError without settling the action promise. _refreshToken's catch/finally never run, so refreshPromise stays pending for the rest of the page lifetime — every subsequent getAccessToken/getAccessTokenSilently/refresh call returns the same hung promise and token refresh on that tab is permanently dead until reload.
• Fix: the constructor now records the initial token but defers scheduleRefresh to the first subscribe() call. Subscribers attach via useSyncExternalStore from effects, which run after hydration, so the router action queue is guaranteed to be initialized before any timer can dispatch a Server Action.
• Nothing is lost for the expired-token case: useAccessToken's mount effect already calls getAccessTokenSilently() post-hydration, which refreshes an expiring token immediately. Background refresh with zero subscribers was already unsupported (unsubscribing the last listener clears the timer), so deferring to first-subscribe matches the store's existing lifecycle.

Closes #463

[greptile-apps]

Greptile Summary

This PR defers initial token-refresh scheduling until the store gains its first subscriber.

• Schedules refresh whenever the subscriber count transitions from zero to one.
• Restores the refresh timer after all consumers unsubscribe and a later consumer mounts.
• Adds constructor and subscription lifecycle tests for expiring and opaque tokens.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains; the zero-to-one subscriber transition restores the timer after remount and resolves the previously reported lifecycle issue.

Important Files Changed

Filename	Overview
src/components/tokenStore.ts	Defers initial refresh scheduling to the first subscriber and correctly restores scheduling after a zero-to-one listener transition.
src/components/tokenStore.spec.ts	Adds coverage for deferred scheduling, multiple subscribers, remount restoration, and opaque initial tokens.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[TokenStore constructed] --> B[Read initial token from cookie]
    B --> C[Do not schedule refresh]
    C --> D[First subscriber attaches after hydration]
    D --> E{Token is a parseable JWT?}
    E -- Yes --> F[Schedule refresh from expiry]
    E -- No --> G[No refresh timer]
    F --> H[Last subscriber detaches]
    H --> I[Clear refresh timer]
    I --> D

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