feat: fetch currency exchange rate

exchange rates are fetched at script startup, and at most once daily, then cached. attribution is added as per ExchangeRate-API's requirements

src/content/content.ts

@@ -9,7 +9,9 @@ import { fetchPrice, fetchKeyPrice, ItemPriceData } from './priceService'
 import { createPriceRow, createStoreButton } from './uiRenderer'
 import { findFirstElement, findFirstChildElement } from './utils/dom'
 import { extractPageTitleFromURL } from './utils/url';
+import { ExchangeRates, prepareExchangeRates } from './exchangeRateService';
 var itemSchema: ItemSchema | null;
+var exchangeRates: ExchangeRates | null;
 
 var locale: string = 'en'
 
@@ -425,7 +427,8 @@ async function inject() {
 		label.style.fontSize = "85%";
 		const updateText = $T("Updated %@.", locale).replace('%@', updateTime.toLocaleString(locale, { year: 'numeric', month: 'long', day: 'numeric', hour: '2-digit', minute: '2-digit', timeZoneName: 'short' }))
 		const attributionText = $T("Trade prices sourced from %@. Currency conversions are approximate.", locale).replace('%@', '<a rel="nofollow" class="external text" href="https://prices.tf">prices.tf</a>');
-		label.innerHTML = `${updateText}<br>${attributionText}`;
+		const exchangeRateAttribution = `<a rel="nofollow" class="external text" href="https://www.exchangerate-api.com">Rates By Exchange Rate API</a>.`;
+		label.innerHTML = `${updateText}<br>${attributionText}<br>${exchangeRateAttribution}`;
 		row.appendChild(label);
 
 		priceProgressRow.insertAdjacentElement('afterend', row);
@@ -440,15 +443,22 @@ function addStyles() {
 	style.innerHTML = styleCss;
 }
 
-prepareSchema().then(function (schema) {
+prepareSchema()
+.then(schema => {
 	itemSchema = schema;
 	if (!itemSchema) {
-		logError("No item schema ready, exiting.");
 		wipeSchema(); // FIXME: ugly hack. requires additional page reload. if prepareSchema returns null, we should handle it properly
-		return;
+		throw new Error("No item schema ready");
 	}
+})
+.then(prepareExchangeRates)
+.then(rates => exchangeRates = rates)
+.then(() => {
 	locale = extractLocaleFromURL(document.URL)
 	addStyles();
 	inject();
 	// TODO: Purge expired price data
-});
+})
+.catch((error) => {
+	logError(error);
+})