Implement environment variable setup for frontend; create .env.production and .env.development files if they don't exist, and update API URLs in frontend components to use dynamic API_BASE_URL for improved configuration management.

frontend/src/views/Invite.vue

@@ -91,7 +91,7 @@ const form = ref({
 // Lade alle Einladungen
 async function loadInvitations() {
   try {
-    const response = await fetch('http://localhost:3010/api/invite', {
+    const response = await fetch('${API_URL}/invite', {
       headers: {
         'Authorization': `Bearer ${authStore.token}`
       }
@@ -116,7 +116,7 @@ async function sendInvite() {
   
   try {
     loading.value = true
-    const response = await fetch('http://localhost:3010/api/invite', {
+    const response = await fetch('${API_URL}/invite', {
       method: 'POST',
       headers: {
         'Content-Type': 'application/json',