Unity login page with MySQL integration

[feason]

Description

• This PR implements a complete user authentication system for Unity, featuring user registration and login functionalities with a MySQL database backend.
• Summary of changes in code:
  • LoginUI.cs: Manages the user interface interactions for both login and registration panels.
  • DatabaseManager.cs: Handles all MySQL database operations, including connection, user existence checks, user creation, and validation, utilizing parameterized queries for security.
  • LoginManager.cs: Implements the core business logic for user login, registration, password hashing (SHA256 + salt), and session management with "remember me" functionality.
  • User.cs: Defines the User data model and UserSession class for managing user state, along with helper methods for input validation.
  • DatabaseSetup.sql: A SQL script to initialize the unity_login_system database and users table, including optional logging and session tables, and pre-populates with test accounts.
  • README.md: Provides comprehensive setup and usage instructions for both MySQL and Unity, covering dependencies, scene setup, and configuration.
  • LoginUISetup.cs: An editor-only script to assist developers in quickly applying consistent styling to the login UI elements.
• Which issues it fixes, if any: None (new feature).

Screenshots/videos:

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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