style admin texts page with card list and form

apply the page shell to the admin texts page and present each
existing text as a card-link plus the new-text form as a card with
a primary submit button. ids (#texts-list, #newTextName, #submit,
#back) and the name attribute on the input are preserved so the
existing cypress flows continue to work.

public/js/texts.js

@@ -8,7 +8,8 @@ document.addEventListener('DOMContentLoaded', () => {
         });
         const texts = await res.json();
         textsList.innerHTML = texts.map(text =>
-            '<li><a href=/admin/texts/'
+            '<li class="card"><a class="card-link"'
+                + ' href=/admin/texts/'
                 + text.id
                 + '>'
                 + text.name
@@ -26,7 +27,9 @@ document.addEventListener('DOMContentLoaded', () => {
         if (res.ok) {
             const text = await res.json();
             const li = document.createElement('li');
+            li.className = 'card';
             const a = document.createElement('a');
+            a.className = 'card-link';
             a.href = '/admin/texts/' + text.id;
             a.textContent = text.name;
             li.appendChild(a);