chore: add AGENTS.md with build, lint, test commands and style guidelines

js/utils/helpers.js

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+export function downloadFile(content, filename, mimeType) {
+    const blob = new Blob([content], { type: mimeType });
+    const url = URL.createObjectURL(blob);
+    const link = document.createElement('a');
+    link.href = url;
+    link.download = filename;
+    document.body.appendChild(link);
+    link.click();
+    document.body.removeChild(link);
+    URL.revokeObjectURL(url);
+}
+
+export function formatDate(date) {
+    return new Date(date).toISOString().slice(0, 16);
+}
+
+export function formatPrice(price, decimals = 2) {
+    return price.toFixed(decimals);
+}
+
+export function formatPercent(value) {
+    return (value >= 0 ? '+' : '') + value.toFixed(2) + '%';
+}

js/utils/index.js

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+export { downloadFile, formatDate, formatPrice, formatPercent } from './helpers.js';