feat: add multi-tenant client management for admins

- Add tenants API endpoints (list, get, create)
- Add tenant middleware override via X-View-Tenant header
- Add TenantSelector dropdown component in header
- Add tenant view store with persistence
- Add Clientes management page
- Update all navigation layouts with Clientes link for admins

Admins can now:
- View list of all clients
- Create new clients with automatic schema setup
- Switch between viewing different clients' data
- See which client they are currently viewing

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/lib/api/tenants.ts

@@ -0,0 +1,36 @@
+import { apiClient } from './client';
+
+export interface Tenant {
+  id: string;
+  nombre: string;
+  rfc: string;
+  plan: string;
+  schemaName: string;
+  createdAt: string;
+  _count?: {
+    users: number;
+  };
+}
+
+export interface CreateTenantData {
+  nombre: string;
+  rfc: string;
+  plan?: 'starter' | 'business' | 'professional' | 'enterprise';
+  cfdiLimit?: number;
+  usersLimit?: number;
+}
+
+export async function getTenants(): Promise<Tenant[]> {
+  const response = await apiClient.get<Tenant[]>('/tenants');
+  return response.data;
+}
+
+export async function getTenant(id: string): Promise<Tenant> {
+  const response = await apiClient.get<Tenant>(`/tenants/${id}`);
+  return response.data;
+}
+
+export async function createTenant(data: CreateTenantData): Promise<Tenant> {
+  const response = await apiClient.post<Tenant>('/tenants', data);
+  return response.data;
+}