This commit introduces the AuthService class, which encapsulates the logic for signing in with Google and GitHub using Firebase Authentication. The class provides methods for initiating the OAuth flows for both providers and for signing out the user.
- Implemented `signInWithGoogle()` to handle Google Sign-In.
- Implemented `signInWithGitHub()` to handle GitHub Sign-In via popup.
- Added `signOut()` method for logging out the user.
- Added `getCurrentUser()` method to fetch the currently authenticated user.
