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/client.ts

@@ -13,6 +13,19 @@ apiClient.interceptors.request.use((config) => {
     if (token) {
       config.headers.Authorization = `Bearer ${token}`;
     }
+
+    // Add viewing tenant header for admin users
+    const tenantViewStore = localStorage.getItem('horux-tenant-view');
+    if (tenantViewStore) {
+      try {
+        const { state } = JSON.parse(tenantViewStore);
+        if (state?.viewingTenantId) {
+          config.headers['X-View-Tenant'] = state.viewingTenantId;
+        }
+      } catch {
+        // Ignore parse errors
+      }
+    }
   }
   return config;
 });