feat: Phase 2 — leadership 大屏 (/overview) + district normalization + drawer a11y

Phase 2 of the UX modernization. Three conflict-free workstreams.

Leadership 驾驶舱 (/overview):
- Wuhan 13-district Leaflet choropleth (public/wuhan_districts.geojson, keyed
  on name, darker=higher per 高风险高亮), legend, hover/click-zoom
- 全部/门诊/住院 Segmented toggle drives choropleth + Top-5 district bar
- literal "数据截至2023-12" as-of badge (D3 honesty); raw spinner → LoadingState
- decompose OverviewDashboard 501→273; 6 components + 2 helpers under components/overview/

District normalization (backend data boundary):
- case_loader.normalize_district + load_cases_by_district_daily collapse the
  26 dirty labels (武昌/武昌区…) → 13 canonical; analysis/grid/insights repointed
  (fixes a grid-merge row-drop bug as a bonus); in-memory, schema unchanged

Shell a11y (code-review carryover):
- drawer is now a proper modal: ESC, body scroll-lock, focus-in + focus-trap
  cycle + focus-restore, role=dialog/aria-modal/aria-label, hamburger aria-expanded
- SideNav expanded state lifted to AppShell so rail+drawer stay in sync
- RouteErrorBoundary around <Outlet/> keeps shell chrome on page/chunk failure

Gates: tsc 0 · vitest 64 · e2e 19/19 (17 user-flows + 2 overview) · build ok
· backend pytest 6 new + 48 regression green

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

frontend/src/components/AppShell.tsx

@@ -1,7 +1,8 @@
-import { useState, useCallback } from 'react';
+import { useState, useCallback, useEffect, useRef } from 'react';
 import { Outlet } from 'react-router-dom';
 import { TopNav } from '@/components/TopNav';
 import { SideNav } from '@/components/SideNav';
+import { RouteErrorBoundary } from '@/components/RouteErrorBoundary';
 import { useRiskStore } from '@/stores';
 import { TESTIDS } from '@/utils/testids';
 
@@ -9,17 +10,84 @@ interface AppShellProps {
   onLogout?: () => void;
 }
 
+// 收集容器内当前可聚焦的元素，供初始聚焦与焦点循环陷阱使用。
+function getFocusable(container: HTMLElement): HTMLElement[] {
+  return Array.from(
+    container.querySelectorAll<HTMLElement>(
+      'a[href], button:not([disabled]), [tabindex]:not([tabindex="-1"])'
+    )
+  ).filter((el) => el.offsetParent !== null || el === document.activeElement);
+}
+
 // 仅负责布局骨架（顶栏 / 侧栏 / 内容区），不涉及路由匹配与鉴权。
 export function AppShell({ onLogout }: AppShellProps) {
   const alerts = useRiskStore((s) => s.alerts);
   const [drawerOpen, setDrawerOpen] = useState(false);
+  // 提升手风琴展开态：导轨与抽屉两份 SideNav 共享，保持同步。
+  const [expandedNav, setExpandedNav] = useState<string | null>('monitoring');
+  const drawerRef = useRef<HTMLElement>(null);
 
   const openDrawer = useCallback(() => setDrawerOpen(true), []);
   const closeDrawer = useCallback(() => setDrawerOpen(false), []);
 
+  // 抽屉作为模态：ESC 关闭、锁定 body 滚动、焦点移入并在关闭后归还给汉堡。
+  useEffect(() => {
+    if (!drawerOpen) return;
+
+    const opener = document.activeElement as HTMLElement | null;
+
+    // 锁定 body 滚动，关闭时还原原值。
+    const prevOverflow = document.body.style.overflow;
+    document.body.style.overflow = 'hidden';
+
+    // 焦点移入抽屉（优先第一个可聚焦元素，否则聚焦抽屉容器本身）。
+    const drawer = drawerRef.current;
+    const focusables = drawer ? getFocusable(drawer) : [];
+    (focusables[0] ?? drawer)?.focus();
+
+    const onKeyDown = (e: KeyboardEvent) => {
+      if (e.key === 'Escape') {
+        e.preventDefault();
+        closeDrawer();
+        return;
+      }
+      // 焦点循环陷阱：Tab 在抽屉内首尾元素之间循环。
+      if (e.key === 'Tab' && drawer) {
+        const items = getFocusable(drawer);
+        if (items.length === 0) {
+          e.preventDefault();
+          drawer.focus();
+          return;
+        }
+        const first = items[0];
+        const last = items[items.length - 1];
+        const active = document.activeElement;
+        if (e.shiftKey && (active === first || active === drawer)) {
+          e.preventDefault();
+          last.focus();
+        } else if (!e.shiftKey && active === last) {
+          e.preventDefault();
+          first.focus();
+        }
+      }
+    };
+
+    document.addEventListener('keydown', onKeyDown);
+
+    return () => {
+      document.removeEventListener('keydown', onKeyDown);
+      document.body.style.overflow = prevOverflow;
+      // 关闭后把焦点还给打开抽屉的元素（汉堡按钮），回退到按 testid 查询。
+      const restoreTarget =
+        opener ??
+        document.querySelector<HTMLElement>(`[data-testid="${TESTIDS.hamburger}"]`);
+      restoreTarget?.focus();
+    };
+  }, [drawerOpen, closeDrawer]);
+
   return (
     <div data-testid={TESTIDS.appShell} className="h-screen bg-bg-page flex flex-col overflow-hidden">
-      <TopNav onLogout={onLogout} onToggleMenu={openDrawer} />
+      <TopNav onLogout={onLogout} onToggleMenu={openDrawer} isMenuOpen={drawerOpen} />
 
       <div className="flex flex-1 min-h-0">
         {/* lg 及以上：持久侧栏导轨 */}
@@ -27,7 +95,11 @@ export function AppShell({ onLogout }: AppShellProps) {
           data-testid={TESTIDS.sidebarRail}
           className="hidden lg:block w-[200px] shrink-0 bg-bg-card border-r border-border"
         >
-          <SideNav alertCount={alerts.length} />
+          <SideNav
+            alertCount={alerts.length}
+            expanded={expandedNav}
+            onExpandedChange={setExpandedNav}
+          />
         </aside>
 
         {/* lg 以下：离屏抽屉 + 遮罩 */}
@@ -39,16 +111,29 @@ export function AppShell({ onLogout }: AppShellProps) {
           />
         )}
         <aside
+          ref={drawerRef}
+          id="app-drawer"
+          role="dialog"
+          aria-modal="true"
+          aria-label="导航菜单"
+          tabIndex={-1}
           data-testid={TESTIDS.appDrawer}
           className={`fixed top-0 left-0 bottom-0 z-50 w-[260px] max-w-[80vw] bg-bg-card border-r border-border shadow-xl transition-transform duration-200 lg:hidden ${
             drawerOpen ? 'translate-x-0' : '-translate-x-full'
           }`}
         >
-          <SideNav alertCount={alerts.length} onNavigate={closeDrawer} />
+          <SideNav
+            alertCount={alerts.length}
+            onNavigate={closeDrawer}
+            expanded={expandedNav}
+            onExpandedChange={setExpandedNav}
+          />
         </aside>
 
         <main className="flex-1 min-w-0 overflow-auto p-5">
-          <Outlet />
+          <RouteErrorBoundary>
+            <Outlet />
+          </RouteErrorBoundary>
         </main>
       </div>
     </div>