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/hosts/HostList.tsx

@@ -231,7 +231,8 @@ export default function HostList(props: HostListProps) {
         columns={columns}
         dataSource={sorted}
         pagination={false}
-        scroll={{ x: 'max-content' }}
+        scroll={{ x: 'max-content', y: 'calc(100vh - 280px)' }}
+        virtual
         rowSelection={{
           selectedRowKeys: selectedGroupIds,
           onChange: (keys) => onSelectionChange(keys as string[]),