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/api/tenants.ts

@@ -0,0 +1,46 @@
+import api from './client'
+
+export interface Tenant {
+  id: string
+  name: string
+  slug: string
+  is_active: boolean
+  user_count?: number
+  created_at: string
+}
+
+export interface TenantCreate {
+  name: string
+  slug: string
+  is_active?: boolean
+}
+
+export interface TenantUpdate {
+  name?: string
+  slug?: string
+  is_active?: boolean
+}
+
+export async function getTenants(): Promise<Tenant[]> {
+  const res = await api.get<Tenant[]>('/tenants')
+  return res.data
+}
+
+export async function getTenant(id: string): Promise<Tenant> {
+  const res = await api.get<Tenant>(`/tenants/${id}`)
+  return res.data
+}
+
+export async function createTenant(data: TenantCreate): Promise<Tenant> {
+  const res = await api.post<Tenant>('/tenants', data)
+  return res.data
+}
+
+export async function updateTenant(id: string, data: TenantUpdate): Promise<Tenant> {
+  const res = await api.patch<Tenant>(`/tenants/${id}`, data)
+  return res.data
+}
+
+export async function deleteTenant(id: string): Promise<void> {
+  await api.delete(`/tenants/${id}`)
+}