Implement Activity Log Pagination and Filter State in BLoC

Add activityLog-specific sub-state fields to AdminPortalState (if using a monolithic BLoC state) or extract an ActivityLogSubState and compose it. Use EventTransformer.droppable() from bloc_concurrency package for the LoadMoreActivities handler to prevent duplicate concurrent fetches. ActivityLogFilterChanged should use restartable() to cancel an in-flight LoadActivityLog triggered by a previous filter change. Store activityLogFilter in state so the UI can reflect current active filters; the filter object should be immutable (use copyWith).

When appending pages, create a new list with List.unmodifiable([...existing, ...newPage]) to maintain state immutability. Consider a private _fetchActivityLogPage() helper to avoid duplicating the fetch + error handling logic across LoadActivityLog and LoadMoreActivities handlers.

Unit tests with bloc_test: (1) LoadActivityLog → loading → loaded with page 1 items, hasMoreActivities: true, (2) LoadActivityLog returns empty → loaded with empty list, hasMoreActivities: false, (3) ActivityLogFilterChanged → clears list → reloads page 1 with new filter, (4) LoadMoreActivities appends page 2 items to existing list, (5) LoadMoreActivities on last page sets hasMoreActivities: false, (6) LoadMoreActivities while fetch in-flight → dropped (assert service called only once), (7) ActivityLogFilterChanged while page fetch in-flight → cancels previous → fetches with new filter, (8) service throws on LoadActivityLog → emits error with previous list preserved. All tests use mock AdminStatisticsService via Mockito.