Make download button work

offscreen.js

@@ -0,0 +1,51 @@
+import { SafariBooksDownloader } from "./safaribooks/downloader.js";
+import { collectSessionCookies } from "./safaribooks/cookies.js";
+
+const downloader = new SafariBooksDownloader((message) => {
+  console.info(`[SafariBooksDownloader] ${message}`);
+});
+
+async function handleOffscreenDownload(message) {
+  const { requestId, bookId, options = {} } = message;
+  if (!bookId) {
+    chrome.runtime.sendMessage({
+      type: "offscreen-download-complete",
+      requestId,
+      ok: false,
+      error: "Missing book ID."
+    });
+    return;
+  }
+
+  try {
+    await collectSessionCookies();
+    await downloader.download(bookId, {
+      theme: options.theme ?? "none",
+      kindle: Boolean(options.kindle)
+    });
+    chrome.runtime.sendMessage({
+      type: "offscreen-download-complete",
+      requestId,
+      ok: true
+    });
+  } catch (error) {
+    chrome.runtime.sendMessage({
+      type: "offscreen-download-complete",
+      requestId,
+      ok: false,
+      error: error.message
+    });
+  }
+}
+
+chrome.runtime.onMessage.addListener((message) => {
+  if (message?.type === "offscreen-download") {
+    handleOffscreenDownload(message);
+  }
+});
+
+chrome.runtime
+  .sendMessage({ type: "offscreen-ready" })
+  .catch(() => {
+    // ignore errors if background not ready yet
+  });