feat: add exercise header with navigation

src/screens/Exercise.tsx

@@ -1,9 +1,22 @@
-import { Center, Text } from "@gluestack-ui/themed";
+import { Center, Icon, Text, VStack } from '@gluestack-ui/themed';
+import { useNavigation } from '@react-navigation/native';
+import { ArrowLeft } from 'lucide-react-native';
+import { TouchableOpacity } from 'react-native';
 
 export function Exercise() {
+  const navigation = useNavigation();
+
+  function handleGoBack() {
+    navigation.goBack();
+  }
+
   return (
-    <Center flex={1}>
-      <Text>Exercise</Text>
-    </Center>
-  )
+    <VStack flex={1}>
+      <VStack px="$8" bg="$gray600" pt="$12">
+        <TouchableOpacity onPress={handleGoBack}>
+          <Icon as={ArrowLeft} color="$green500" size="xl" />
+        </TouchableOpacity>
+      </VStack>
+    </VStack>
+  );
 }