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/TopNav.tsx

@@ -5,6 +5,8 @@ interface TopNavProps {
   onLogout?: () => void;
   // 移动端汉堡按钮：切换侧栏抽屉。
   onToggleMenu?: () => void;
+  // 抽屉是否展开（用于汉堡按钮的 aria-expanded）。
+  isMenuOpen?: boolean;
 }
 
 function Clock() {
@@ -16,7 +18,7 @@ function Clock() {
   return <span>{time.toLocaleTimeString('zh-CN', { hour: '2-digit', minute: '2-digit' })}</span>;
 }
 
-export function TopNav({ onLogout, onToggleMenu }: TopNavProps) {
+export function TopNav({ onLogout, onToggleMenu, isMenuOpen = false }: TopNavProps) {
   return (
     <nav className="h-[52px] bg-bg-card border-b border-border flex items-center px-5 z-50">
       {onToggleMenu && (
@@ -24,6 +26,8 @@ export function TopNav({ onLogout, onToggleMenu }: TopNavProps) {
           type="button"
           onClick={onToggleMenu}
           aria-label="打开菜单"
+          aria-expanded={isMenuOpen}
+          aria-controls="app-drawer"
           data-testid={TESTIDS.hamburger}
           className="lg:hidden mr-3 -ml-1 w-9 h-9 flex items-center justify-center rounded-md text-text-secondary hover:bg-bg-hover transition-colors"
         >