Replace the setState-driven data flow with useSyncExternalStore so the
drawer reads cached work-item details synchronously on first render.
Warm reopens now paint the cached content immediately with zero blank
flash. Adds a pub/sub layer (subscribeWorkItemDetailsCache /
notifyWorkItemDetailsCache) to all cache-write paths so React is
notified on every touch or invalidation. Includes design doc at
docs/gh-work-item-drawer-cache-flash.md.
Co-authored-by: Orca <help@stably.ai>
