Initialize navigation with AuthenticationFragment

Also remove unused MainFragment-related code

app/src/main/java/gq/kirmanak/mealie/ui/auth/AuthenticationFragment.kt

@@ -0,0 +1,28 @@
+package gq.kirmanak.mealie.ui.auth
+
+import android.os.Bundle
+import android.view.LayoutInflater
+import android.view.View
+import android.view.ViewGroup
+import androidx.fragment.app.Fragment
+import gq.kirmanak.mealie.databinding.FragmentAuthenticationBinding
+
+class AuthenticationFragment : Fragment() {
+    private var _binding: FragmentAuthenticationBinding? = null
+    private val binding: FragmentAuthenticationBinding
+        get() = checkNotNull(_binding) { "Binding requested when fragment is off screen" }
+
+    override fun onCreateView(
+        inflater: LayoutInflater,
+        container: ViewGroup?,
+        savedInstanceState: Bundle?
+    ): View {
+        _binding = FragmentAuthenticationBinding.inflate(inflater, container, false)
+        return binding.root
+    }
+
+    override fun onDestroyView() {
+        super.onDestroyView()
+        _binding = null
+    }
+}