Restore enterprise/LDAP login mode detection

- Added /users/auth-mode public endpoint to detect LDAP status
- Updated frontend to call getAuthMode() instead of hardcoded false
- Login page now automatically switches to enterprise mode when LDAP is enabled
- Fixes missing LDAP user login UI that was previously disabled with TODO comment

This restores the functionality that was commented out earlier where the login
page would auto-detect whether to show local user list or enterprise username input.

frontend/lib/api.ts

@@ -85,6 +85,12 @@ axiosInstance.interceptors.response.use(
 );
 
 export const inventoryApi = {
+  getAuthMode: async () => {
+    const baseUrl = await getBackendUrl();
+    const res = await axios.get(`${baseUrl}/users/auth-mode`);
+    return res.data;
+  },
+
   getItems: async () => {
     const res = await axiosInstance.get('/items/');
     return res.data;