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/app/globals.css

@@ -5,7 +5,7 @@
 @tailwind utilities;
 
 @layer base {
-    :root {
+        :root {
     /* Base colors from DESIGN.md - AUTO-GENERATED from colors.mjs */
 
     /* Base colors from DESIGN.md */
@@ -87,6 +87,8 @@
   }
 
 
+
+
   * {
     @apply border-border;
     border-radius: 0 !important;