Implement Anonymous/Guest Sign-In Functionality

[pandaxin8]

Description: The application currently supports sign-in with email and password. We aim to enhance user accessibility by introducing an anonymous or guest sign-in option. This feature will allow users to explore the game without creating an account, providing them with immediate access to gameplay while still preserving the option to sign in or sign up for a more personalized experience later.

Acceptance Criteria:

• Implement an anonymous sign-in method using Firebase Authentication.
• Ensure that users who choose to sign in anonymously can access the game's features similar to signed-in users.
• Provide an option for anonymous users to convert their account into a permanent account (email/password) without losing game progress.
• Update the SignInScreen UI to include a button or option for users to continue as a guest.
• Ensure that the game state is properly managed and loaded for anonymous users, similar to authenticated users.
• Conduct thorough testing to verify that anonymous users can sign in, play the game, and optionally convert to a full account seamlessly.

[pandaxin8]

Separating "Provide an option for anonymous users to convert their account into a permanent account (email/password) without losing game progress." into a separate issue: ##53