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.

.claude/settings.local.json

@@ -155,7 +155,12 @@
       "Bash(pkill -9 -f \"uvicorn backend.main\")",
       "Bash(pkill -9 -f \"python3.*start_servers\")",
       "Bash(curl -v -X POST http://localhost:8916/users/login -H 'Content-Type: application/json' -d '{\"username\":\"admin\",\"password\":\"admin\"}')",
-      "Bash(pkill -9 -f \"uvicorn\\\\|gunicorn\")"
+      "Bash(pkill -9 -f \"uvicorn\\\\|gunicorn\")",
+      "Bash(curl -s http://localhost:3000)",
+      "Bash(/data/programare_AI/tfm_ainventory/venv/bin/python3 -m uvicorn backend.main:app --host 0.0.0.0 --port 8916)",
+      "Bash(curl -s http://localhost:8916/users/)",
+      "Bash(pkill -9 -f \"uvicorn.*backend.main\" sleep 2 /data/programare_AI/tfm_ainventory/venv/bin/python3 -m uvicorn backend.main:app --host 0.0.0.0 --port 8916)",
+      "Bash(curl -s http://localhost:8916/users/auth-mode)"
     ]
   }
 }