Memoize groupActivitiesByDate in the social ActivityFeed

[jaynomyaro]

Description
This PR introduces memoization for the groupActivitiesByDate function used in the Social ActivityFeed component. By caching results based on input dependencies, we reduce redundant computations and improve rendering performance, especially when the feed contains a large number of activities.

Key Changes
Wrapped groupActivitiesByDate with useMemo to avoid recalculations when inputs remain unchanged.

Ensured dependency array includes only relevant props/state to prevent stale data.

Added unit tests to confirm memoization behavior and correctness of grouped results.

Verified that UI updates correctly when new activities are added or existing ones change.

Motivation
The ActivityFeed often re-renders due to state changes, causing groupActivitiesByDate to run multiple times unnecessarily. Memoization improves efficiency and responsiveness, leading to a smoother user experience.

Testing
Confirmed that repeated renders with identical activity lists do not trigger recomputation.

Verified that changes in activities correctly invalidate the memoized result.

Benchmarked rendering performance before and after memoization.

Checklist
[x] Memoization implemented

[x] Dependencies correctly set

[x] Tests updated and passing

[x] Performance validated
Closed #977

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