feat(B3): add tenant management UI (CRUD + tenant selector)

- frontend/src/api/tenants.ts: Tenant CRUD API client (getTenants, getTenant, createTenant, updateTenant, deleteTenant)
- frontend/src/pages/Tenants.tsx: Admin page with table, create/edit modals, delete confirm, and cross-tenant selector persisted in localStorage
- App.tsx: /tenants route (AdminRoute-guarded)
- Layout.tsx: Tenants sidebar link (admin-only, Building2 icon)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -16,6 +16,7 @@ import ProductDetailPage from './pages/ProductDetail'
 import NewProductOrderPage from './pages/NewProductOrder'
 import NotificationsPage from './pages/Notifications'
 import PreferencesPage from './pages/Preferences'
+import TenantsPage from './pages/Tenants'
 
 function ProtectedRoute({ children }: { children: React.ReactNode }) {
   const token = useAuthStore((s) => s.token)
@@ -57,6 +58,14 @@ export default function App() {
               </AdminRoute>
             }
           />
+          <Route
+            path="tenants"
+            element={
+              <AdminRoute>
+                <TenantsPage />
+              </AdminRoute>
+            }
+          />
           <Route path="materials" element={<MaterialsPage />} />
           <Route path="activity" element={<WorkerActivityPage />} />
           <Route path="products" element={<ProductLibraryPage />} />