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/components/layout/Layout.tsx

@@ -1,5 +1,5 @@
 import { Outlet, NavLink, useNavigate, Link } from 'react-router-dom'
-import { LayoutDashboard, Package, Settings, LogOut, FlaskConical, Activity, Library, Plus, SlidersHorizontal } from 'lucide-react'
+import { LayoutDashboard, Package, Settings, LogOut, FlaskConical, Activity, Library, Plus, SlidersHorizontal, Building2 } from 'lucide-react'
 import { useAuthStore } from '../../store/auth'
 import { clsx } from 'clsx'
 import { useQuery } from '@tanstack/react-query'
@@ -120,6 +120,22 @@ export default function Layout() {
               Admin
             </NavLink>
           )}
+          {user?.role === 'admin' && (
+            <NavLink
+              to="/tenants"
+              className={({ isActive }) =>
+                clsx(
+                  'flex items-center gap-3 px-3 py-2 rounded-md text-sm font-medium transition-colors',
+                  isActive
+                    ? 'bg-accent-light text-accent'
+                    : 'text-content-secondary hover:bg-surface-hover',
+                )
+              }
+            >
+              <Building2 size={18} />
+              Tenants
+            </NavLink>
+          )}
         </nav>
 
         <div className="p-3 border-t border-border-default space-y-1">