Add auth navigations

src/screens/Signup.tsx

@@ -0,0 +1,23 @@
+import React from 'react'
+import {Text, Button, View, SafeAreaView} from 'react-native'
+import type {RootStackScreenProps} from '../routes/types'
+import {useStores} from '../state'
+
+export function Signup({navigation}: RootStackScreenProps<'Signup'>) {
+  const store = useStores()
+  return (
+    <SafeAreaView style={{flex: 1}}>
+      <View style={{flex: 1}}>
+        <Text>Let's create your account</Text>
+        <Button
+          title="Create new account"
+          onPress={() => store.session.setAuthed(true)}
+        />
+        <Button
+          title="Log in to an existing account"
+          onPress={() => navigation.navigate('Login')}
+        />
+      </View>
+    </SafeAreaView>
+  )
+}