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/header.tsx

@@ -3,6 +3,7 @@
 import { useThemeStore } from '@/stores/theme-store';
 import { themes, type ThemeName } from '@/themes';
 import { Button } from '@/components/ui/button';
+import { TenantSelector } from '@/components/tenant-selector';
 import { Sun, Moon, Palette } from 'lucide-react';
 
 const themeIcons: Record<ThemeName, React.ReactNode> = {
@@ -27,7 +28,8 @@ export function Header({ title }: { title: string }) {
     <header className="sticky top-0 z-30 flex h-16 items-center justify-between border-b bg-background/95 backdrop-blur px-6">
       <h1 className="text-xl font-semibold">{title}</h1>
 
-      <div className="flex items-center gap-2">
+      <div className="flex items-center gap-3">
+        <TenantSelector />
         <Button
           variant="ghost"
           size="icon"