feat: Phase 3 — 视角/perspective presets + URL-driven granularity + doctor privacy

Phase 3 of the UX modernization. Frontend-only role view-presets (D2 — no
backend, no JWT role claim, honestly labeled 视角 not 权限). Three conflict-free lanes.

Role infrastructure (worker-session):
- sessionStore with single swappable getRoleSource() seam (localStorage today,
  one-line swap to /api/auth/me for future RBAC); Role = official|community|doctor|admin
- 视角 switcher in TopNav (replaces hardcoded "admin"); on change persists role +
  navigates to that perspective's default landing
- roleViews.ts: ROLE_LABELS + roleDefaultPath (official→/overview?granularity=district,
  community→/monitoring?granularity=street, doctor→/alerts?view=cluster, admin→/monitoring)
- RoleRedirect index route → current role's default; * fallback unchanged

URL-driven granularity (worker-monitoring):
- granularity (city|district|street) query param is the source of truth; drilldownStore
  DERIVES from it via a one-way effect; deep-linkable + reload-safe
- reconciled the imperative desync — DistrictBreakdown no longer calls
  useDrilldownStore.getState(); MonitoringDashboard owns useSearchParams, writes URL
- granularity-control Segmented (全市/区域/街道); district-rollup testid

Role-aware alerts + privacy (worker-alerts):
- doctor/cluster view: individual alert markers hard-locked off (effective flag is
  single source of truth; toggle not rendered) → aggregated density only; DiseaseFilter
  mounted; privacy invariant testable via hidden patient-point DOM mirror (count=0)
- 官员: 100m grid hidden (grid-layer-wrapper unrendered); admin/community unchanged

Gates: tsc 0 · vitest 75 · e2e 30/30 (incl 10 new P3 tests) · build ok

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

frontend/src/components/TopNav.tsx

@@ -1,5 +1,8 @@
 import { useState, useEffect } from 'react';
+import { useNavigate } from 'react-router-dom';
 import { TESTIDS } from '@/utils/testids';
+import { useSessionStore, ROLES, type Role } from '@/stores/sessionStore';
+import { ROLE_LABELS, roleDefaultPath } from '@/utils/roleViews';
 
 interface TopNavProps {
   onLogout?: () => void;
@@ -18,6 +21,38 @@ function Clock() {
   return <span>{time.toLocaleTimeString('zh-CN', { hour: '2-digit', minute: '2-digit' })}</span>;
 }
 
+// 视角切换器：纯前端视图预设（D2）。刻意标注「视角」而非「权限」——不是访问控制。
+// 切换时持久化角色并跳转到该视角的默认落地页。
+function PerspectiveSwitcher() {
+  const role = useSessionStore((s) => s.role);
+  const setRole = useSessionStore((s) => s.setRole);
+  const navigate = useNavigate();
+
+  const handleChange = (next: Role) => {
+    setRole(next);
+    navigate(roleDefaultPath(next));
+  };
+
+  return (
+    <label className="flex items-center gap-1.5 text-[13px] text-text-secondary">
+      <span className="text-text-muted hidden sm:inline">视角</span>
+      <select
+        data-testid={TESTIDS.perspectiveSwitcher}
+        value={role}
+        onChange={(e) => handleChange(e.target.value as Role)}
+        aria-label="切换视角"
+        className="bg-bg-card border border-border rounded-md px-2 py-1 text-[13px] text-text-primary hover:bg-bg-hover focus:outline-none focus:ring-1 focus:ring-primary cursor-pointer"
+      >
+        {ROLES.map((r) => (
+          <option key={r} value={r} data-testid={`${TESTIDS.perspectiveOption}-${r}`}>
+            {ROLE_LABELS[r]}
+          </option>
+        ))}
+      </select>
+    </label>
+  );
+}
+
 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">
@@ -62,7 +97,7 @@ export function TopNav({ onLogout, onToggleMenu, isMenuOpen = false }: TopNavPro
           <svg className="w-4 h-4" fill="currentColor" viewBox="0 0 24 24">
             <path d="M12 12c2.21 0 4-1.79 4-4s-1.79-4-4-4-4 1.79-4 4 1.79 4 4 4zm0 2c-2.67 0-8 1.34-8 4v2h16v-2c0-2.66-5.33-4-8-4z" />
           </svg>
-          admin
+          <PerspectiveSwitcher />
         </div>
         {onLogout && (
           <button