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/components/layouts/sidebar.tsx

@@ -13,6 +13,7 @@ import {
   Calendar,
   Bell,
   Users,
+  Building2,
 } from 'lucide-react';
 import { useAuthStore } from '@/stores/auth-store';
 import { logout } from '@/lib/api/auth';
@@ -29,6 +30,10 @@ const navigation = [
   { name: 'Configuracion', href: '/configuracion', icon: Settings },
 ];
 
+const adminNavigation = [
+  { name: 'Clientes', href: '/clientes', icon: Building2 },
+];
+
 export function Sidebar() {
   const pathname = usePathname();
   const router = useRouter();
@@ -45,6 +50,10 @@ export function Sidebar() {
     }
   };
 
+  const allNavigation = user?.role === 'admin'
+    ? [...navigation.slice(0, -1), ...adminNavigation, navigation[navigation.length - 1]]
+    : navigation;
+
   return (
     <aside className="fixed left-0 top-0 z-40 h-screen w-64 border-r bg-card">
       <div className="flex h-full flex-col">
@@ -60,7 +69,7 @@ export function Sidebar() {
 
         {/* Navigation */}
         <nav className="flex-1 space-y-1 px-3 py-4">
-          {navigation.map((item) => {
+          {allNavigation.map((item) => {
             const isActive = pathname === item.href || pathname.startsWith(`${item.href}/`);
             return (
               <Link