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/admin/apropos-groups-form.php

@@ -6,7 +6,7 @@
  *   $groups      array   Existing groups data
  */
 ?>
-<form action="/admin/actions/apropos.php" method="post" class="admin-form" id="apropos-form-<?= $aproposKey ?>">
+<form action="/admin/actions/apropos.php" method="post" class="admin-form" id="apropos-form-<?= $aproposKey ?>" data-autosave>
     <input type="hidden" name="csrf_token" value="<?= htmlspecialchars($_SESSION['csrf_token']) ?>">
     <input type="hidden" name="apropos_key" value="<?= htmlspecialchars($aproposKey) ?>">
 
@@ -55,10 +55,7 @@
     <button type="button" class="btn btn--primary add-group-btn-f"
             data-key="<?= $aproposKey ?>">+ Ajouter un contact</button>
 
-    <div class="admin-form-footer">
-        <button type="submit" class="btn btn--primary">Enregistrer</button>
-        <a href="/admin/contenus.php" class="btn btn--secondary admin-cancel-link">Annuler</a>
-    </div>
+    <div class="autosave-status" data-autosave-status></div>
 
     <template id="entry-template-f-<?= $aproposKey ?>">
         <div class="apropos-entry">