Fix the UI dying at load, and add a page smoke test

$('#prefs').onclick referenced a prefsModal that was never defined, and an
uncaught ReferenceError stops the whole script -- taking the theme toggle,
the feed filter, the event stream and loadFeeds() down with it, so the app
rendered an empty shell.

The scheduling patch had anchored on a function the rewrite already
deleted; str.replace matched nothing and said nothing.

tests/page-smoke.js executes the page against a stub DOM so this class of
failure is visible, since every server-side check passed while the UI was
completely dead. Handler wiring now skips a bad reference instead of
throwing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AdXho5tTkjFLeUXKbEjKBh

tests/page-smoke.js

@@ -0,0 +1,66 @@
+// Executes web/index.html's script against a stub DOM and fails on anything thrown.
+//
+// This exists because a ReferenceError at load once blanked the whole UI: a patch
+// anchored on a function that no longer existed, so `prefsModal` was referenced but
+// never defined. `node --check` passes that happily -- it is a parse, not a run --
+// and every server-side test passed too, because the server was fine.
+//
+//   node tests/page-smoke.js
+const fs = require('fs');
+const path = require('path');
+const vm = require('vm');
+
+const html = fs.readFileSync(path.join(__dirname, '..', 'web', 'index.html'), 'utf8');
+const script = html.split('<script>')[1].split('</script>')[0];
+const ids = new Set([...html.matchAll(/id="([^"]+)"/g)].map(m => m[1]));
+
+const missing = [];
+const el = (name) => new Proxy({ style: {}, dataset: {}, classList: { add(){}, remove(){}, toggle(){}, contains(){ return false; } },
+  value: '', textContent: '', innerHTML: '', hidden: false, children: [], firstElementChild: null,
+  appendChild(){}, removeChild(){}, remove(){}, insertAdjacentHTML(){}, addEventListener(){},
+  setAttribute(){}, getAttribute(){ return null; }, select(){}, setSelectionRange(){}, focus(){},
+  replaceWith(){}, querySelector(){ return el('nested'); }, querySelectorAll(){ return []; },
+  play(){ return Promise.resolve(); }, pause(){}, closest(){ return null; } },
+  { get: (t, k) => k in t ? t[k] : undefined, set: (t, k, v) => (t[k] = v, true) });
+
+const document = {
+  querySelector(sel) {
+    if (sel.startsWith('#') && !ids.has(sel.slice(1))) { missing.push(sel); return null; }
+    return el(sel);
+  },
+  querySelectorAll: () => [],
+  createElement: () => el('created'),
+  addEventListener(){}, body: el('body'),
+  documentElement: { dataset: {} },
+};
+
+const ctx = {
+  document, console,
+  window: { isSecureContext: false, addEventListener(){} },
+  localStorage: { getItem: () => null, setItem(){}, removeItem(){} },
+  navigator: { clipboard: undefined, sendBeacon(){}, mediaSession: undefined },
+  fetch: () => Promise.resolve({ ok: true, status: 200, json: () => Promise.resolve([]), text: () => Promise.resolve('') }),
+  EventSource: function () { this.close = () => {}; },
+  MediaMetadata: function () {},
+  Blob: function () {},
+  setTimeout, clearTimeout, setInterval, clearInterval,
+  confirm: () => false, prompt: () => null, alert(){},
+  Date, Math, JSON, Object, Array, String, Number, Promise, Error, FormData: function(){},
+  URLSearchParams, encodeURIComponent, decodeURIComponent, parseInt, parseFloat, isNaN,
+};
+ctx.globalThis = ctx;
+ctx.window.location = { href: '' };
+
+try {
+  vm.createContext(ctx);
+  vm.runInContext(script, ctx, { filename: 'index.html<script>', timeout: 5000 });
+} catch (e) {
+  console.error('FAIL: the page script threw while loading\n  ' + e.stack.split('\n').slice(0, 3).join('\n  '));
+  process.exit(1);
+}
+
+if (missing.length) {
+  console.error('FAIL: handlers wired to elements that do not exist: ' + [...new Set(missing)].join(', '));
+  process.exit(1);
+}
+console.log('OK: page script loads clean, every selector it wires at load exists');