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. The approval window now posts an
empty message on its approval port every 20 seconds while it is open;
each one is an event, which keeps the worker running.

tests/e2e/slowApproval.js, run by make test-e2e, leaves a transaction
and a signature approval open for seven minutes and requires the site
to receive both results. It drives Chromium over the DevTools pipe
without Playwright, because Playwright keeps DevTools attached to the
worker and Chrome then never stops it.

Model: opus-5-5

.gitea/workflows/e2e.yml

@@ -35,10 +35,10 @@ on: [push]
 jobs:
     e2e-chrome:
         runs-on: ubuntu-latest
-        # Bounds the image build, a cold cache included, and both Chrome
+        # Bounds the image build, a cold cache included, and the three Chrome
         # programs, so a hung browser frees the shared runner. README.md
         # "In CI" has the measured times.
-        timeout-minutes: 20
+        timeout-minutes: 40
         steps:
             # actions/checkout v4.2.2, 2026-02-22
             - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683