feat(nav): palette en icône près du titre, sections accordéon (retour #16)

Le bouton Recherche & commandes devient une icône 🔍 seule à
droite du titre (sous le logo en rail replié). Les groupes
deviennent des accordéons: chevron animé, transition douce,
sections fermées mémorisées (appliquées avant le rendu), section
de la page courante toujours ouverte; rail replié = icônes.

static/js/nav.js

@@ -42,4 +42,47 @@
             } catch (e) { /* private mode: state lasts for the page only */ }
         });
     }
+
+    /* Accordion sections: the closed set lives in body classes
+       (applied pre-paint by the bootstrap script in _header.html)
+       and is persisted here on every toggle. */
+    var groupBtns = document.querySelectorAll('[data-group-btn]');
+    function persistClosedGroups() {
+        var closed = [];
+        groupBtns.forEach(function (btn) {
+            if (document.body.classList.contains('nav-closed-' + btn.getAttribute('data-group-btn'))) {
+                closed.push(btn.getAttribute('data-group-btn'));
+            }
+        });
+        try {
+            localStorage.setItem('kanbante-nav-closed', JSON.stringify(closed));
+        } catch (e) { /* non-fatal */ }
+    }
+    function syncGroupBtn(btn) {
+        var closed = document.body.classList.contains(
+            'nav-closed-' + btn.getAttribute('data-group-btn'));
+        btn.setAttribute('aria-expanded', closed ? 'false' : 'true');
+    }
+    groupBtns.forEach(function (btn) {
+        syncGroupBtn(btn);
+        btn.addEventListener('click', function () {
+            document.body.classList.toggle(
+                'nav-closed-' + btn.getAttribute('data-group-btn'));
+            syncGroupBtn(btn);
+            persistClosedGroups();
+        });
+    });
+    /* The section holding the current page is always opened. */
+    var activeLink = document.querySelector('.sidebar .side-link.active');
+    if (activeLink) {
+        var section = activeLink.closest('.side-section');
+        if (section) {
+            var key = section.getAttribute('data-group');
+            if (document.body.classList.contains('nav-closed-' + key)) {
+                document.body.classList.remove('nav-closed-' + key);
+                groupBtns.forEach(syncGroupBtn);
+                persistClosedGroups();
+            }
+        }
+    }
 })();