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/stores/tenant-view-store.ts

@@ -0,0 +1,23 @@
+import { create } from 'zustand';
+import { persist } from 'zustand/middleware';
+
+interface TenantViewState {
+  viewingTenantId: string | null;
+  viewingTenantName: string | null;
+  setViewingTenant: (id: string | null, name: string | null) => void;
+  clearViewingTenant: () => void;
+}
+
+export const useTenantViewStore = create<TenantViewState>()(
+  persist(
+    (set) => ({
+      viewingTenantId: null,
+      viewingTenantName: null,
+      setViewingTenant: (id, name) => set({ viewingTenantId: id, viewingTenantName: name }),
+      clearViewingTenant: () => set({ viewingTenantId: null, viewingTenantName: null }),
+    }),
+    {
+      name: 'horux-tenant-view',
+    }
+  )
+);