[CRX] Set manifest_version to 3

- Replace DOM-based pdfHandler.html (background page) with background.js
  (extension service worker).

- Adjust logic of background scripts to account for the fact that the
  scripts can execute repeatedly during a browser session. Primarily,
  register relevant extension event handlers at the top level and use
  in-memory storage.session API to keep track of initialization state.

- Extension URL router: replace blocking webRequest with the service
  worker-specific "fetch" event.

- PDF detection: replace blocking webRequest with declarativeNetRequest.
  This requires Chrome 128+. The next commit will add a fallback for
  earlier Chrome versions.

web/chromecom.js

@@ -31,7 +31,11 @@ if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("CHROME")) {
   // is rewritten as soon as possible.
   const queryString = document.location.search.slice(1);
   const m = /(^|&)file=([^&]*)/.exec(queryString);
-  const defaultUrl = m ? decodeURIComponent(m[2]) : "";
+  let defaultUrl = m ? decodeURIComponent(m[2]) : "";
+  if (!defaultUrl && queryString.startsWith("DNR:")) {
+    // Redirected via DNR, see registerPdfRedirectRule in pdfHandler.js.
+    defaultUrl = queryString.slice(4);
+  }
 
   // Example: chrome-extension://.../http://example.com/file.pdf
   const humanReadableUrl = "/" + defaultUrl + location.hash;