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/overview/AlertSeverityDonut.tsx

@@ -0,0 +1,67 @@
+import { memo } from 'react';
+import { PieChart, Pie, Cell, Tooltip, Legend, ResponsiveContainer } from 'recharts';
+import { EmptyState } from '@/components/ui';
+import { CHART_COLORS } from './chartColors';
+
+export interface AlertSlice {
+  name: string;
+  value: number;
+  color: string;
+}
+
+interface AlertSeverityDonutProps {
+  data: AlertSlice[];
+}
+
+const tooltipStyle = {
+  backgroundColor: '#FFFFFF',
+  border: `1px solid ${CHART_COLORS.tooltipBorder}`,
+  borderRadius: '8px',
+  fontSize: '12px',
+};
+
+function AlertSeverityDonutComponent({ data }: AlertSeverityDonutProps) {
+  const hasData = data.some((d) => d.value > 0);
+
+  return (
+    <div className="card p-4">
+      <div className="text-[11px] font-medium text-text-secondary uppercase tracking-wide mb-4">
+        预警严重度分布
+      </div>
+      {hasData ? (
+        <div className="flex items-center justify-center">
+          <ResponsiveContainer width="100%" height={240}>
+            <PieChart>
+              <Pie
+                data={data}
+                cx="50%"
+                cy="50%"
+                innerRadius={50}
+                outerRadius={80}
+                paddingAngle={4}
+                dataKey="value"
+                nameKey="name"
+              >
+                {data.map((entry) => (
+                  <Cell key={entry.name} fill={entry.color} />
+                ))}
+              </Pie>
+              <Tooltip
+                contentStyle={tooltipStyle}
+                formatter={(value: number, name: string) => [value, name]}
+              />
+              <Legend
+                wrapperStyle={{ fontSize: '12px' }}
+                formatter={(value: string) => <span className="text-text-primary">{value}</span>}
+              />
+            </PieChart>
+          </ResponsiveContainer>
+        </div>
+      ) : (
+        <EmptyState title="暂无预警数据" />
+      )}
+    </div>
+  );
+}
+
+export const AlertSeverityDonut = memo(AlertSeverityDonutComponent);