auth-gate: Adding Login Gate and Account Management to DeepSeek Harness Web Interface

`@dsh-login-gate/auth-gate` is a plugin bundle designed to provide access control and account management for the DeepSeek Harness Web interface. It is suitable for shared machines or LAN environments and requires no modifications to the DSH core. When not logged in, a fullscreen login page covers the entire application; after logging in, users can modify their username and password or log out via the "Account Settings" page within System Settings.