A missing page answers the site's 404 page in the Frame (#248)

ddiff --git a/server/app.js b/server/app.js
iindex 09c0a68..5d9302e 100644
---- a/server/app.js
++++ b/server/app.js
@@@ -237,7 +237,8 @@ function createApp({ config, db, registry, pool, quotas, cache, runs, auth, keys
      // A browser asking for a page that is not here gets a page; an API client the problem document.
      app.use((req, res) => {
          if (req.method === 'GET' && !req.path.startsWith('/api/') && !req.path.startsWith('/v1/') && req.accepts(['application/json', 'text/html']) === 'text/html') {
--            return res.status(404).type('html').set('Content-Security-Policy', home.HOME_CSP).set('Cache-Control', 'no-store').send(home.renderNotFound());
++            return res.status(404).type('html').set('Content-Security-Policy', home.HOME_CSP).set('Cache-Control', 'no-store')
++                .set('X-Robots-Tag', 'noindex').send(home.renderNotFound({ siteUrl: config.baseUrl }));
          }
          return http.sendProblem(res, 404, 'ai.not_found', { detail: `no route ${req.method} ${req.path}`, ctx: req.ov });
      });
ddiff --git a/server/home.js b/server/home.js
iindex f581c9d..428d4bf 100644
---- a/server/home.js
++++ b/server/home.js
@@@ -414,12 +414,35 @@ function llmsTxt({ siteUrl }) {
      ].join('\n');
  }
  
--/** The page a browser gets for a path nothing serves: a real document (language, title, the app icon), not JSON. */
--function renderNotFound() {
--    return ['<!doctype html>', '<html lang="en">', '<head>', '<meta charset="utf-8">', '<meta name="viewport" content="width=device-width, initial-scale=1">',
--        `<title>Not found · ${SITE_NAME}</title>`, '<meta name="robots" content="noindex">', appIcon.headTags({ site: 'ai' }), '</head>',
--        '<body style="font-family:system-ui,sans-serif;margin:3rem auto;max-width:36rem;padding:0 1rem;line-height:1.5">',
--        '<h1>Not found</h1>', `<p>Nothing is here. <a href="/">${SITE_NAME}</a> · <a href="/stats">Prices and latency</a> · <a href="${NETWORK_URL}">OpenVibe</a></p>`, '</body>', '</html>'].join('\n');
++/**
++ * The page a browser gets for a path nothing serves: the site's own Frame, one way back, and no clue
++ * about what was asked for. An API client keeps the problem document (server/app.js).
++ */
++function renderNotFound({ siteUrl = 'https://ai.openvibe.services' } = {}) {
++    const host = String(siteUrl).replace(/^https?:\/\//, '').replace(/\/.*$/, '');
++    return shell.page({
++        name: SITE_NAME, service: 'ai', lang: 'en',
++        title: `Page not found · ${SITE_NAME}`,
++        siteName: SITE_NAME,
++        description: `There is no page at this address on ${host}.`,
++        robots: 'noindex, follow',
++        navbar: { service: 'ai', apiBase: NETWORK_URL, links: NAV_LINKS, history: { type: 'page', title: SITE_NAME } },
++        footer: FOOTER, home: '/', navLinks: NAV_LINKS,
++        head: [
++            appIcon.headTags({ site: 'ai' }),
++            `<link rel="stylesheet" href="${ovServe.url(showcase.STYLESHEET)}">`,
++        ].join('\n'),
++        body: `<div id="navbar-mount"></div>
++<main id="main" class="page">
++<h1>Page not found</h1>
++<p>There is no page at this address on ${esc(host)}. The link may be old or mistyped.</p>
++<div class="sc-actions">
++<a class="sc-btn sc-primary" href="/">${SITE_NAME} home</a>
++<a class="sc-btn" href="/stats">Prices and latency</a>
++<a class="sc-btn" href="${GUIDE}">Developer guide</a>
++</div>
++</main>`,
++    });
  }
  
  module.exports = { renderHome, renderUpdates, renderStats, llmsTxt, llmsFullTxt, PUBLIC_PAGES, HOME_CSP, SITE_NAME, renderNotFound, FAQ, DESCRIPTION };
ddiff --git a/test/home.test.js b/test/home.test.js
iindex 3aaca46..8e91a06 100644
---- a/test/home.test.js
++++ b/test/home.test.js
@@@ -183,13 +183,22 @@ t.test('the CSPs let Cloudflare Web Analytics load and report (it injects its be
      assert.strictEqual(stats.headers.get('content-security-policy'), HOME_CSP);
  });
  
--t.test('a browser gets a page for a path nothing serves; an API client the problem document; /favicon.ico is the icon', async () => {
++t.test('a browser that asks for a missing page gets the site 404 page; an API client the problem document', async () => {
      const page = await request(h.base, 'GET', '/no-such-page', { headers: { Accept: BROWSER } });
      assert.strictEqual(page.status, 404);
--    assert.ok(page.text.includes('<html lang="en">') && page.text.includes('<title>Not found') && page.text.includes('rel="icon"'));
++    assert.match(page.headers.get('content-type'), /^text\/html/);
++    assert.ok(page.text.includes('<h1>Page not found</h1>'));
++    assert.ok(page.text.includes('There is no page at this address on ai.openvibe.services. The link may be old or mistyped.'));
++    assert.ok(page.text.includes('id="navbar-mount"'), 'the page is in the site Frame');
++    assert.ok(page.text.includes('<meta name="robots" content="noindex, follow">'));
++    assert.strictEqual(page.headers.get('x-robots-tag'), 'noindex');
++    assert.ok(page.text.includes('class="sc-btn sc-primary" href="/"'), 'home is the first way back');
      const api = await request(h.base, 'GET', '/api/v1/no-such-thing', { headers: { Accept: BROWSER } });
      assert.strictEqual(api.status, 404);
      assert.match(api.headers.get('content-type'), /json/);
++    const notHtml = await request(h.base, 'GET', '/no-such-page', { headers: { Accept: 'application/json' } });
++    assert.strictEqual(notHtml.status, 404);
++    assert.match(notHtml.headers.get('content-type'), /json/);
      const icon = await request(h.base, 'GET', '/favicon.ico');
      assert.strictEqual(icon.status, 200);
      assert.match(icon.headers.get('content-type'), /^image\/svg\+xml/);