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/nodes/NodeList.tsx

@@ -648,7 +648,8 @@ export default function NodeList({
           columns={columns}
           pagination={false}
           loading={loading}
-          scroll={{ x: 'max-content' }}
+          scroll={{ x: 'max-content', y: 'calc(100vh - 320px)' }}
+          virtual
           size="middle"
           rowKey="key"
           rowSelection={dataSource.length > 1 ? {