feat: surface detailed download progress

Changelog:
- forward download progress from background/offscreen workers to content frames
- add progress UI and button state management in Safari Books page script
- emit structured progress callbacks and faster image fetching in SafariBooksDownloader
- sync Firefox bundle with progress plumbing and concurrency improvements
- ignore packaged extension archives

Description:
Expose progress events through the downloader so the content script can present real-time status, including chapter and asset fetching milestones. Added concurrency controls, page detection, and packaging telemetry, while keeping Firefox build behavior in sync and ignoring generated .xpi bundles.

firefox/background.js

@@ -2,27 +2,94 @@ import { SafariBooksDownloader } from "./safaribooks/downloader.js";
 import { collectSessionCookies } from "./safaribooks/cookies.js";
 
 const runtime = typeof browser !== "undefined" ? browser : chrome;
+const pendingDownloads = new Map();
 
 const downloader = new SafariBooksDownloader((message) => {
   console.log(`[BackgroundDownloader] ${message}`);
+}, (payload) => {
+  const entry = payload?.bookId ? pendingDownloads.get(payload.bookId) : null;
+  if (!entry) {
+    return;
+  }
+  sendProgress(entry, payload);
 });
 
-runtime.runtime.onMessage.addListener(async (message) => {
-  if (message?.type !== "downloadBook") {
+function sendProgress(entry, payload) {
+  if (!entry?.tabId) {
+    return;
+  }
+  const progressMessage = {
+    type: "download-progress",
+    payload
+  };
+
+  try {
+    if (typeof browser !== "undefined" && browser.tabs?.sendMessage) {
+      browser.tabs.sendMessage(entry.tabId, progressMessage).catch(() => {});
+    } else {
+      chrome.tabs.sendMessage(entry.tabId, progressMessage, () => {
+        const error = chrome.runtime.lastError;
+        if (error) {
+          console.debug("Progress send error", error.message);
+        }
+      });
+    }
+  } catch (error) {
+    console.debug("Unable to dispatch progress message", error);
+  }
+}
+
+runtime.runtime.onMessage.addListener((message, sender, sendResponse) => {
+  if (!message || message.type !== "downloadBook") {
     return undefined;
   }
 
-  const { bookId, theme = "none", kindle = false } = message;
+  const bookId = message.bookId;
   if (!bookId) {
-    return { ok: false, error: "Missing book ID." };
+    sendResponse({ ok: false, error: "Missing book ID." });
+    return false;
   }
 
+  if (pendingDownloads.has(bookId)) {
+    sendResponse({ ok: false, error: "Download already in progress." });
+    return false;
+  }
+
+  const entry = {
+    bookId,
+    options: {
+      theme: message.theme ?? "none",
+      kindle: Boolean(message.kindle)
+    },
+    tabId: sender?.tab?.id ?? null,
+    sendResponse
+  };
+
+  pendingDownloads.set(bookId, entry);
+  startDownload(entry).catch((error) => {
+    console.error("[BackgroundDownloader] unexpected failure", error);
+    finalizeDownload(entry, { ok: false, error: error.message || String(error) });
+  });
+
+  return true;
+});
+
+async function startDownload(entry) {
   try {
     await collectSessionCookies();
-    await downloader.download(bookId, { theme, kindle });
-    return { ok: true };
+    await downloader.download(entry.bookId, entry.options);
+    finalizeDownload(entry, { ok: true });
   } catch (error) {
     console.error("[BackgroundDownloader] Download failed:", error);
-    return { ok: false, error: error.message };
+    finalizeDownload(entry, { ok: false, error: error.message || String(error) });
   }
-});
+}
+
+function finalizeDownload(entry, response) {
+  pendingDownloads.delete(entry.bookId);
+  try {
+    entry.sendResponse?.(response);
+  } catch (error) {
+    console.debug("sendResponse failed", error);
+  }
+}