T-16: Centralize PAGE_SIZE frontend constants

- Add BAN_PAGE_SIZE (100) and HISTORY_PAGE_SIZE (50) to frontend/src/utils/constants.ts
- Replace local PAGE_SIZE definitions in useBans.ts and HistoryPage.tsx with imports
- Eliminates risk of pagination constants silently diverging from backend defaults
- Single source of truth for all pagination sizes

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/src/utils/constants.ts

@@ -15,3 +15,9 @@ export const TIME_RANGE_LABELS: Record<TimeRange, string> = {
   "30d": "Last 30 days",
   "365d": "Last 365 days",
 } as const;
+
+/** Items per page for the ban dashboard table. */
+export const BAN_PAGE_SIZE = 100 as const;
+
+/** Items per page for the history table. */
+export const HISTORY_PAGE_SIZE = 50 as const;