A missing page answers the site's 404 page in the Frame (#147)

ddiff --git a/server/app.js b/server/app.js
iindex acf6672..ab4d0c2 100644
---- a/server/app.js
++++ b/server/app.js
@@@ -163,8 +163,11 @@ function createApp(opts = {}) {
      if (opts.accountData) app.post('/internal/events', opts.accountData.consumer({ secrets: [config.events.secret].filter((s) => s.length >= 32), send: opts.accountSend || null }));
  
      app.use((req, res) => {
--        if (req.path.startsWith('/api/') || req.path.startsWith('/internal/')) return http.sendProblem(res, 404, 'not_found', { ctx: req.ov });
--        return res.status(404).type('text/plain').send('Not found\n');
++        if (req.path.startsWith('/api/') || req.path.startsWith('/internal/') || req.accepts(['application/json', 'text/html']) !== 'text/html') {
++            return http.sendProblem(res, 404, 'not_found', { ctx: req.ov });
++        }
++        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({ config }));
      });
      // eslint-disable-next-line no-unused-vars
      app.use((err, req, res, next) => {
ddiff --git a/server/web/home.js b/server/web/home.js
iindex 452f31c..876690d 100644
---- a/server/web/home.js
++++ b/server/web/home.js
@@@ -112,6 +112,33 @@ function renderUpdates({ config }) {
      });
  }
  
++/** 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. */
++function renderNotFound({ config }) {
++    const host = String(config.baseUrl).replace(/^https?:\/\//, '').replace(/\/.*$/, '');
++    const links = [{ label: 'How it works', href: '/#how' }, { label: 'API', href: '/#api' }];
++    return shell.page({
++        name: SITE_NAME, service: 'run', lang: 'en',
++        title: `Page not found · ${SITE_NAME}`,
++        siteName: SITE_NAME, description: `There is no page at this address on ${host}.`,
++        robots: 'noindex, follow',
++        navbar: { service: 'run', apiBase: NETWORK_URL, links, history: { type: 'page', title: SITE_NAME } },
++        footer: FOOTER, home: '/', navLinks: links,
++        head: [appIcon.headTags({ site: 'run' }), '<link rel="manifest" href="/manifest.webmanifest">',
++            `<link rel="apple-touch-icon" href="${require('openvibe-shared/app-icons').url('run', 192)}">`,
++            `<link rel="stylesheet" href="${ovServe.url(showcase.STYLESHEET)}">`, UPDATES_FONT].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="/#how">How it works</a>
++<a class="sc-btn" href="/#api">API</a>
++</div>
++</main>`,
++    });
++}
++
  const SDK_EXAMPLE = `const { createClient } = require('openvibe-sdk/core');
  const { createRunClient } = require('openvibe-sdk/run');
  
@@@ -212,4 +239,4 @@ function body() {
      });
  }
  
--module.exports = { renderHome, renderUpdates, HOME_CSP, SITE_NAME, DESCRIPTION, FAQ };
++module.exports = { renderHome, renderUpdates, renderNotFound, HOME_CSP, SITE_NAME, DESCRIPTION, FAQ };
ddiff --git a/test/site.test.js b/test/site.test.js
iindex 24bcb85..b5fd81b 100644
---- a/test/site.test.js
++++ b/test/site.test.js
@@@ -58,6 +58,20 @@ const { boot, check, done } = require('./helpers/app');
              assert.strictEqual(r.status, 404);
              assert.match(r.headers.get('content-type'), /problem\+json/);
          });
++        await check('a browser that asks for a missing page gets the site 404 page in the Frame', async () => {
++            const r = await t.call('GET', '/no-such-page-xyz', { token: null, headers: { Accept: 'text/html' } });
++            assert.strictEqual(r.status, 404);
++            assert.match(r.headers.get('content-type'), /^text\/html/);
++            assert.ok(r.text.includes('<h1>Page not found</h1>'), r.text.slice(0, 300));
++            assert.ok(r.text.includes('There is no page at this address on run.test. The link may be old or mistyped.'));
++            assert.ok(r.text.includes('id="navbar-mount"'), 'the page is in the site Frame');
++            assert.ok(r.text.includes('<meta name="robots" content="noindex, follow">'));
++            assert.strictEqual(r.headers.get('x-robots-tag'), 'noindex');
++            assert.ok(r.text.includes('class="sc-btn sc-primary" href="/"'), 'home is the first way back');
++            const api = await t.call('GET', '/no-such-page-xyz', { token: null, headers: { Accept: 'application/json' } });
++            assert.strictEqual(api.status, 404);
++            assert.match(api.headers.get('content-type'), /problem\+json/);
++        });
      } finally {
          await t.close();
      }