Replace cache-level optimistic updates (onMutate with temp IDs) with
TQ v5's UI-level pattern (useMutationState + render-time derivation)
for both issue-level and comment-level reaction toggles.
The cache-level approach caused a race condition: temp IDs in the cache
couldn't be deduplicated against real IDs from WS events, causing
reaction counts to briefly flash incorrect values (e.g. 0→1→2→1).
The UI pattern keeps the cache clean (always server-confirmed data) and
derives optimistic state at render time from pending mutation variables.
WS events safely update the cache without conflicting with temp data.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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