feat: Refactor BillingManager to improve validation, add filter functionality, and enhance UI with new icons and export feature

backend/server.js

@@ -343,8 +343,8 @@ app.post('/api/billing', async (req, res) => {
     // Validate each billing item has required fields
     for (let i = 0; i < billingData.length; i++) {
         const item = billingData[i];
-        if (!item.hostName || !item.nodeName || !item.country || !item.provider) {
-            return res.status(400).send(`Billing item at index ${i} is missing required fields`);
+        if (!item.hostName || !item.country || !item.provider) {
+            return res.status(400).send(`Billing item at index ${i} is missing required fields: hostName, country, provider`);
         }
     }