fix: a slow answer to an approval still reaches the site (closes #540)

Chrome stopped the background worker about six minutes after a site's
request even with the approval window open, and the approval and the
site's waiting request went with it. Every approval is now added through
addPendingApproval(), which starts a timer in the worker that calls
runtime.getPlatformInfo() every 20 seconds while any approval is
pending; Chrome counts each call as activity.

tests/e2e/slowApproval.js, run by make test-e2e, leaves a transaction
and a signature approval open for seven minutes in minimized windows and
requires the site to receive both results. It drives Chromium over the
DevTools pipe because Playwright keeps DevTools attached to the worker.
Its loopback dApp server, shared with the Firefox suite, moves to
tests/e2e/dapp.js.

Model: opus-5-5

src/shared/browserApi.js

@@ -145,6 +145,14 @@ function sendMessage(message) {
     return invoke(runtimeApi(), "sendMessage", message);
 }
 
+/**
+ * @returns {Promise<Object>} the operating system and processor the browser
+ *     runs on.
+ */
+function getPlatformInfo() {
+    return invoke(runtimeApi(), "getPlatformInfo");
+}
+
 /**
  * Send a message nobody is expected to answer, and swallow the fact that
  * nobody did.
@@ -262,6 +270,7 @@ module.exports = {
     actionApi,
     alarmsApi,
     extensionApi,
+    getPlatformInfo,
     hasBrowserNamespace,
     notify,
     runtimeApi,