perf(frontend): replace blank Suspense fallbacks with Spin, switch to matchMedia hook, add virtual table scrolling (#6187)

- routes.tsx, LazyMount.tsx: replace Suspense fallback={null} with Spin
  loader so page transitions and lazy modals never show blank content
- useMediaQuery.ts: switch from resize event to matchMedia change event,
  eliminating state updates on every pixel drag; export MOBILE_BREAKPOINT_PX
- SubPage.tsx: drop duplicate inline isMobile logic (7 lines), use shared
  useMediaQuery(576) (2 lines)
- ClientsPage, InboundList, HostList, NodeList: add virtual + scroll.y to
  Table for viewport-only DOM rendering of large datasets

frontend/src/pages/clients/ClientsPage.tsx

@@ -1350,7 +1350,8 @@ export default function ClientsPage() {
                           rowSelection={rowSelection}
                           pagination={tablePagination}
                           size="small"
-                          scroll={{ x: 1200 }}
+                          scroll={{ x: 1200, y: 'calc(100vh - 380px)' }}
+                          virtual
                           onChange={onTableChange}
                           locale={{
                             emptyText: (