fix(spa): set api url to constant

spa/src/components/Converter.vue

@@ -1,6 +1,8 @@
 <script setup>
 import { ref, watch, reactive } from 'vue'
 
+const url = 'http://localhost:3000'
+
 const result = ref(null)
 const number_to_convert = ref(null)
 const error = ref(false)
@@ -32,7 +34,7 @@ async function convertNumber() {
   }
 
   try {
-    const res = await fetch('http://127.0.0.1:3000', options)
+    const res = await fetch(url, options)
     const roman = await res.text()
     result.value = roman
     last_results.push({ arabic: number_to_convert.value, roman })