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/EmptyState.tsx

@@ -0,0 +1,29 @@
+import React, { memo } from 'react';
+
+export const EmptyState = memo(function EmptyState({
+  title,
+  description,
+  icon,
+  action,
+}: {
+  title: string;
+  description?: string;
+  icon?: React.ReactNode;
+  action?: React.ReactNode;
+}): JSX.Element {
+  return (
+    <div
+      data-testid="empty-state"
+      className="flex flex-col items-center justify-center gap-3 py-12 px-6 text-center"
+    >
+      {icon && (
+        <div className="text-text-muted text-4xl">{icon}</div>
+      )}
+      <p className="text-sm font-medium text-text-secondary">{title}</p>
+      {description && (
+        <p className="text-xs text-text-muted max-w-xs">{description}</p>
+      )}
+      {action && <div className="mt-2">{action}</div>}
+    </div>
+  );
+});