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/hooks/useBans.ts

@@ -8,11 +8,9 @@
 import { useCallback, useEffect, useRef, useState } from "react";
 import { fetchBans } from "../api/dashboard";
 import { handleFetchError } from "../utils/fetchError";
+import { BAN_PAGE_SIZE } from "../utils/constants";
 import type { DashboardBanItem, TimeRange, BanOriginFilter } from "../types/ban";
 
-/** Items per page for the ban table. */
-const PAGE_SIZE = 100;
-
 /** Return value shape for {@link useBans}. */
 export interface UseBansResult {
   /** Ban items for the current page. */
@@ -67,7 +65,7 @@ export function useBans(
     setError(null);
 
     try {
-      const data = await fetchBans(timeRange, page, PAGE_SIZE, origin, source, controller.signal);
+      const data = await fetchBans(timeRange, page, BAN_PAGE_SIZE, origin, source, controller.signal);
       if (controller.signal.aborted) return;
       setBanItems(data.items);
       setTotal(data.total);