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/Calendar.vue

@@ -101,7 +101,7 @@ async function loadCalendar() {
     error.value = null
     
     const response = await fetch(
-      `http://localhost:3010/api/calendar?year=${selectedYear.value}&month=${selectedMonth.value}`,
+      `${API_URL}/calendar?year=${selectedYear.value}&month=${selectedMonth.value}`,
       {
         headers: {
           'Authorization': `Bearer ${authStore.token}`