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/pages/ReportsCenter.tsx

@@ -1,4 +1,5 @@
 import { useEffect, useState, useCallback } from 'react';
+import { LoadingState } from '@/components/ui';
 import { BarChart, Bar, XAxis, YAxis, CartesianGrid, Tooltip, ResponsiveContainer } from 'recharts';
 import { FileText, Download, Activity, TrendingUp, TrendingDown, AlertTriangle, ChevronLeft, RefreshCw } from 'lucide-react';
 import { useReportsStore } from '@/stores/reportsStore';
@@ -94,7 +95,7 @@ function ReportList({ onSelect }: { onSelect: (id: string) => void }) {
       </div>
 
       {isLoading ? (
-        <div className="text-center py-8 text-sm text-gray-500">加载中...</div>
+        <LoadingState />
       ) : reports.length === 0 ? (
         <div className="text-center py-12">
           <FileText className="w-12 h-12 text-gray-300 mx-auto mb-3" />
@@ -329,7 +330,7 @@ export function ReportsCenter() {
   }, [fetchReportsList]);
 
   return (
-    <div>
+    <div data-testid="page-reports">
       {error && view === 'list' && (
         <ErrorBanner error={error} onRetry={() => { clearError(); fetchReportsList(); }} onDismiss={clearError} />
       )}