Add auth navigations

src/screens/Login.tsx

@@ -0,0 +1,17 @@
+import React from 'react'
+import {Text, Button, View, SafeAreaView} from 'react-native'
+import type {RootStackScreenProps} from '../routes/types'
+import {useStores} from '../state'
+
+export function Login({navigation}: RootStackScreenProps<'Login'>) {
+  const store = useStores()
+  return (
+    <SafeAreaView style={{flex: 1}}>
+      <View style={{flex: 1}}>
+        <Text>Welcome! Time to sign in</Text>
+        <Button title="Login" onPress={() => store.session.setAuthed(true)} />
+        <Button title="Sign Up" onPress={() => navigation.navigate('Signup')} />
+      </View>
+    </SafeAreaView>
+  )
+}