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/js/app/autosave.js

@@ -0,0 +1,76 @@
+/**
+ * Auto-save for admin content edit forms.
+ *
+ * Watches forms with [data-autosave] attribute. Debounces 1.5s after the
+ * last change, POSTs to the form's action, and shows a small status bar.
+ *
+ * The status indicator lives in a sibling element with [data-autosave-status].
+ * States: idle → saving… → saved ✓ / error !
+ */
+(() => {
+  const forms = document.querySelectorAll('form[data-autosave]');
+  if (!forms.length) return;
+
+  const DEBOUNCE_MS = 1500;
+
+  for (const form of forms) {
+    const statusEl = document.querySelector(form.dataset.autosaveStatus || '[data-autosave-status]');
+    let timer = null;
+    let dirty = false;
+
+    const setStatus = (text, cls) => {
+      if (!statusEl) return;
+      statusEl.textContent = text;
+      statusEl.className = 'autosave-status ' + (cls || '');
+    };
+
+    const doSave = () => {
+      if (!dirty) return;
+      dirty = false;
+      setStatus('Enregistrement…', 'autosave-status--saving');
+
+      const formData = new FormData(form);
+
+      fetch(form.action, {
+        method: 'POST',
+        body: formData,
+        headers: { 'Accept': 'application/json' },
+      })
+        .then((r) => {
+          if (!r.ok) throw new Error('HTTP ' + r.status);
+          setStatus('Enregistré ✓', 'autosave-status--saved');
+          // Refresh CSRF token from response if provided
+          r.json().then((data) => {
+            if (data && data.csrf_token) {
+              const csrfInput = form.querySelector('input[name="csrf_token"]');
+              if (csrfInput) csrfInput.value = data.csrf_token;
+              const meta = document.querySelector('meta[name="csrf-token"]');
+              if (meta) meta.setAttribute('content', data.csrf_token);
+            }
+          }).catch(() => {});
+        })
+        .catch(() => {
+          setStatus('Erreur !', 'autosave-status--error');
+          dirty = true;
+        });
+    };
+
+    const schedule = () => {
+      dirty = true;
+      setStatus('', '');
+      clearTimeout(timer);
+      timer = setTimeout(doSave, DEBOUNCE_MS);
+    };
+
+    // Watch all inputs inside the form
+    form.addEventListener('input', schedule);
+    form.addEventListener('change', schedule);
+
+    // Clear dirty on manual submit
+    form.addEventListener('submit', () => {
+      clearTimeout(timer);
+      dirty = false;
+      setStatus('', '');
+    });
+  }
+})();