Add configurable user registration via login screen

AISix currently does not provide a configurable option for enabling or disabling user registration directly from the login screen (cloud on-premise). This improvement would allow administrators to decide whether new users can self‑register or if registration should be restricted, making the system more flexible for different deployment scenarios.

Motivation

• Provide control over whether self‑registration is allowed
• Improve security in environments where only invited users should access
• Simplify onboarding in open environments where registration is desired

Proposed Implementation

• Add a toggle in the Settings screen:
  • Enable user registration (default: on)
• When enabled, display a Sign Up option on the login screen
• When disabled, hide the option and restrict access to existing accounts only
• Ensure clear error messages if registration is attempted while disabled

Acceptance Criteria

• Configurable toggle for user registration in UI
• Sign Up option visible only when enabled
• Registration blocked when disabled
• Documentation updated with configuration instructions

Start by tracing the existing Settings screen, login screen, and user-registration flow in the Rust project, then identify where configuration and registration access are handled. Implement the toggle and ensure the Sign Up option and registration behavior follow it, with a clear disabled-state error. Update the documentation with configuration instructions and verify each acceptance criterion.