Add the web interface

Plain HTML, CSS and JavaScript modules, with no build step. Hash routes
and relative URLs make it work under any path. The job list is polled:
every 2 seconds while a job runs, and less often otherwise.

tests/js/startup-error.test.mjs

@@ -0,0 +1,14 @@
+import assert from 'node:assert/strict';
+import { test } from 'node:test';
+import { install, server, settle } from './flow-helpers.mjs';
+
+const ui = install();
+
+test('startup failure reports the API error rather than leaving a blank application', async (t) => {
+  const backend = server(t);
+  backend.on('GET', 'auth/check', { error: 'Service unavailable' }, 503);
+  await import('../../bindery/static/js/main.js');
+  await settle();
+  assert.match(ui.app.textContent, /Service unavailable/);
+  assert.deepEqual(backend.requests.map((r) => r.path), ['auth/check']);
+});