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/admin.css

@@ -2140,3 +2140,63 @@ th.admin-ap-col {
   width: 100%;
   box-sizing: border-box;
 }
+
+/* ── Sidebar links editor ──────────────────────────────────────────────── */
+
+.sidebar-link-row {
+  display: flex;
+  align-items: flex-start;
+  gap: var(--space-xs);
+  margin-bottom: var(--space-s);
+}
+
+.sidebar-link-fields {
+  display: grid;
+  grid-template-columns: 1fr 2fr;
+  gap: var(--space-s);
+  flex: 1;
+}
+
+.sidebar-link-fields > div {
+  display: flex;
+  flex-direction: column;
+}
+
+.sidebar-link-fields label {
+  font-size: var(--step--1);
+  margin-bottom: var(--space-3xs);
+}
+
+.sidebar-link-fields input {
+  width: 100%;
+  box-sizing: border-box;
+}
+
+.sidebar-link-row .admin-icon-btn--delete {
+  margin-top: calc(var(--step--1) + var(--space-3xs) + 0.3em);
+  flex-shrink: 0;
+}
+
+/* ── Auto-save status indicator ────────────────────────────────────────── */
+
+.autosave-status {
+  font-size: var(--step--2);
+  min-height: 1.5em;
+  margin-top: var(--space-2xs);
+  color: var(--text-tertiary);
+  transition: color 0.2s;
+}
+
+.autosave-status--saving {
+  color: var(--accent-yellow);
+  font-style: italic;
+}
+
+.autosave-status--saved {
+  color: var(--accent-green);
+}
+
+.autosave-status--error {
+  color: var(--error);
+  font-weight: 500;
+}