roi: save CX in localStorage

ts/roi.ts

@@ -11,6 +11,12 @@ async function getROI(cx: string) {
 const lowVolume = document.querySelector('input#low-volume') as HTMLInputElement;
 
 const cxSelect = document.querySelector('select#cx') as HTMLSelectElement;
+{
+	const savedCx = localStorage.getItem('cx');
+	if (savedCx !== null)
+		cxSelect.value = savedCx;
+}
+
 const expertise = {
 	AGRICULTURE: 'agri',
 	CHEMISTRY: 'chem',
@@ -107,6 +113,7 @@ async function render() {
 	}
 	document.getElementById('last-updated')!.textContent =
 		`last updated: ${lastModified.toLocaleString(undefined, {dateStyle: 'full', timeStyle: 'long', hour12: false})}`;
+	localStorage.setItem('cx', cx);
 }
 
 function color(n: number, low: number, high: number): string {