feat(frontend): Phase 1 UX foundation — react-router v6 + responsive AppShell

Atomic foundation for the consensus-approved UX modernization (makes the
platform URL-addressable, refresh-safe, and mobile-usable for hospital demos).

- Migrate hand-rolled useState page switching → react-router v6 (routes.tsx,
  thin App.tsx auth gate, NavLink SideNav, lazy+Suspense per route)
- Add responsive AppShell: persistent rail (lg:) ⇄ off-canvas drawer + hamburger
  (<lg); kills hardcoded ml-[200px]; usable at 375px
- Add ui primitive kit (Skeleton/LoadingState/EmptyState/Card/Panel/Segmented)
- Sweep all raw "加载中..." text loaders → skeleton primitives (G4)
- Centralize test ids (utils/testids.ts); rewrite e2e for URL nav (17/17 pass:
  deep-link, refresh-preserves-page, back, 375px drawer/no-scroll)
- Harden DemographicAnalysis against API shape mismatch (defensive normalize)
- gitignore playwright-report/ and test-results/

Gates: tsc --noEmit 0 · pnpm build ok · e2e 17/17 · grep 加载中 zero outside ui/

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/ui/Skeleton.tsx

@@ -0,0 +1,22 @@
+import { memo } from 'react';
+
+export const Skeleton = memo(function Skeleton({
+  className,
+  rounded,
+}: {
+  className?: string;
+  rounded?: boolean;
+}): JSX.Element {
+  return (
+    <div
+      data-testid="skeleton"
+      className={[
+        'animate-pulse bg-bg-hover',
+        rounded ? 'rounded-full' : 'rounded',
+        className,
+      ]
+        .filter(Boolean)
+        .join(' ')}
+    />
+  );
+});