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/public/assets/css/apropos.css

@@ -71,35 +71,23 @@
     border-left-color: var(--accent-primary);
 }
 
-.apropos-toc-erg {
+.apropos-toc-link:first-of-type {
     padding-top: var(--space-s);
     border-top: 1px solid var(--border-primary);
 }
 
-.apropos-toc-erg a {
-    font-size: var(--step--2);
-    color: var(--accent-primary);
-    text-decoration: none;
-    transition: opacity 0.15s;
-}
-
-.apropos-toc-erg a:hover {
-    color: var(--accent-primary);
-    opacity: 1;
-}
-
-.apropos-toc-source {
+.apropos-toc-link + .apropos-toc-link {
     padding-top: var(--space-xs);
 }
 
-.apropos-toc-source a {
+.apropos-toc-link a {
     font-size: var(--step--2);
     color: var(--accent-primary);
     text-decoration: none;
     transition: opacity 0.15s;
 }
 
-.apropos-toc-source a:hover {
+.apropos-toc-link a:hover {
     color: var(--accent-primary);
     opacity: 1;
 }
@@ -326,7 +314,7 @@
         padding-left: 0;
     }
 
-    .apropos-toc-erg {
+    .apropos-toc-link {
         border-top: none;
         padding-top: 0;
         margin-left: auto;