Rework contenus-edit: auto-save, OverType toolbar, dynamic sidebar links

- Auto-save: new autosave.js with 1.5s debounce, watches all forms with
  data-autosave, POSTs to form action with Accept: application/json, shows
  saving/saved/error status indicator
- All action handlers (page.php, apropos.php, form-help.php) now detect
  JSON Accept header and return {success, csrf_token} or {error} responses
- OverType toolbar enabled (toolbar:true) on all three markdown editors
  (page, about_page, form_help)
- Sidebar links: replaced fixed erg_site_url / source_code_url rows with
  dynamic sidebar_links array of {label, url} objects. Add/remove via JS.
  Fallback migration reads legacy keys if sidebar_links is empty.
- Updated AboutController and about.php template to render dynamic links
- Updated apropos.css: unified .apropos-toc-link replacing .apropos-toc-erg
  and .apropos-toc-source
- New CSS: autosave-status states, sidebar-link-row layout
- Removed all Enregistrer + Annuler buttons — auto-save and h1 back-arrow
  make them redundant

app/templates/public/about.php

@@ -44,16 +44,15 @@ function renderEntries(array $entries): string
                 <?php endif; ?>
                 <li><a href="#apropos-credits">Crédits</a></li>
             </ul>
-            <div class="apropos-toc-erg">
-                <a href="<?= htmlspecialchars($ergSiteUrl ?: 'https://erg.be') ?>" target="_blank" rel="noopener">
-                    Site de l'erg ↗
-                </a>
-            </div>
-            <div class="apropos-toc-source">
-                <a href="<?= htmlspecialchars($sourceCodeUrl ?: 'https://git.erg.school/PostERG/xamxam') ?>" target="_blank" rel="noopener">
-                    Code source ↗
+            <?php if (!empty($sidebarLinks)): ?>
+            <?php foreach ($sidebarLinks as $sl): ?>
+            <div class="apropos-toc-link">
+                <a href="<?= htmlspecialchars($sl['url'] ?? '#') ?>" target="_blank" rel="noopener">
+                    <?= htmlspecialchars($sl['label'] ?? 'Lien') ?> ↗
                 </a>
             </div>
+            <?php endforeach; ?>
+            <?php endif; ?>
         </nav>
 
         <!-- MIDDLE: main prose + sections -->