Guide the public Chat message composer (#520)

ddiff --git a/server/web/shell.js b/server/web/shell.js
iindex c75ae22..4d35fea 100644
---- a/server/web/shell.js
++++ b/server/web/shell.js
@@@ -227,10 +227,11 @@ ${youBar(o.you || {})}`;
   * o: { action, label, placeholder, max, add: the panel Add opens (pictureComposer's), hint }
   */
  function composer(o = {}) {
++    const questHook = o.action === '/send' ? ' data-ov-guide="chat.message"' : '';
      const add = o.add || '';
      return `<div class="oc-dock${add ? ' has-add' : ''}"><form class="oc-compose" method="post" action="${esc(o.action || '')}" id="oc-compose"><label for="oc-input" class="oc-sr">${esc(o.label || 'Message')}</label>
  <div class="oc-compose-box"><div class="oc-compose-tray" data-oc-tray hidden></div>
--<div class="oc-compose-row">${add ? `<button type="button" class="oc-compose-add" data-oc-attach aria-expanded="false" aria-controls="oc-add-panel" aria-label="Add a picture" title="Add a picture">${IC.plus}</button>` : ''}<textarea id="oc-input" name="message" maxlength="${Number(o.max) || 2000}" rows="1" required placeholder="${esc(o.placeholder || 'Write a message')}"></textarea><button type="submit" class="oc-send" aria-label="Send" title="Send">${IC.send}</button></div></div></form>${add}</div>`;
++<div class="oc-compose-row">${add ? `<button type="button" class="oc-compose-add" data-oc-attach aria-expanded="false" aria-controls="oc-add-panel" aria-label="Add a picture" title="Add a picture">${IC.plus}</button>` : ''}<textarea id="oc-input" name="message" maxlength="${Number(o.max) || 2000}" rows="1" required placeholder="${esc(o.placeholder || 'Write a message')}"></textarea><button type="submit" class="oc-send"${questHook} aria-label="Send" title="Send">${IC.send}</button></div></div></form>${add}</div>`;
  }
  
  /** The you-bar: your mark, your status, mute, deafen and settings (the foot of the side column). */
ddiff --git a/test/web.test.js b/test/web.test.js
iindex 565441f..60a4c67 100644
---- a/test/web.test.js
++++ b/test/web.test.js
@@@ -54,6 +54,7 @@ t('global chat reads without JavaScript; signing in shows the composer', async (
      assert.ok(!/id="oc-compose"/.test(r.text), 'no composer for a guest');
      const me = await req('GET', '/', { headers: cookie(alice) });
      assert.match(me.text, /id="oc-compose"/);
++    assert.match(me.text, /data-ov-guide="chat.message"/, 'public composer exposes its quest control');
      // Under the room, what openvibe.chat does: six cards, each a place to go, with the showcase stylesheet. A guest's
      // messages and settings cards go through sign-in; a signed-in person's go straight there.
      const what = (/<section class="sc-sec" id="what-chat-does"[\s\S]*?<\/section>/.exec(r.text) || [''])[0];