Initialize navigation with AuthenticationFragment

Also remove unused MainFragment-related code

app/src/main/java/gq/kirmanak/mealie/MainActivity.kt

@@ -1,18 +1,13 @@
 package gq.kirmanak.mealie
 
-import androidx.appcompat.app.AppCompatActivity
 import android.os.Bundle
-import gq.kirmanak.mealie.ui.main.MainFragment
+import androidx.appcompat.app.AppCompatActivity
+import gq.kirmanak.mealie.databinding.MainActivityBinding
 
 class MainActivity : AppCompatActivity() {
-
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
-        setContentView(R.layout.main_activity)
-        if (savedInstanceState == null) {
-            supportFragmentManager.beginTransaction()
-                .replace(R.id.container, MainFragment.newInstance())
-                .commitNow()
-        }
+        val binding = MainActivityBinding.inflate(layoutInflater)
+        setContentView(binding.root)
     }
 }