Add an Onglets (Tabs) widget

New widget where each tab is a real "conteneur" element posed as a child
(see screens/elements/add_tab.py) — this reuses everything that already
exists for a normal container (adding a Répéteur/Conteneur/etc. inside via
"Ajouter DANS ce conteneur", renaming to change the tab's visible label,
deleting via the standard trash icon) instead of inventing a separate
storage format for tabs.

The widget's own properties panel gets a dedicated "Onglets" section to
add a tab, rename one, jump to its content, or delete it. Rendering
(render_onglets.py) builds a tab bar + one panel per tab, switched
client-side (forgeShowTab, in both screen_edit.html and play.html) with
only one panel visible at a time.

Distinct from the existing "activer_onglet" flow action (2.3, manual
show-one/hide-siblings) — that stays available for custom show/hide
wiring; this widget is the turnkey version with tab management built in.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

screens/widgets/registry.py

@@ -277,4 +277,14 @@ WIDGETS = {
             c_slider("arrondi", "Arrondi des coins de la boîte", "attr:_arrondi", unit="", min=0, max=40, default=12, group="Apparence"),
         ],
     },
+    "onglets": {
+        # Chaque onglet est un vrai élément "conteneur" posé comme enfant
+        # (voir screens/elements/add_tab.py et render_onglets.py) — pas de
+        # gestion dédiée ici : ajouter/renommer/supprimer un onglet se fait
+        # via le bloc "Onglets" du panneau de propriétés (screen_edit.html),
+        # qui réutilise les routes existantes (element_rename, element_delete).
+        "label": "Onglets", "icon": "🗂️", "tag": "div", "has_content": False, "special_render": "onglets",
+        "w": 60, "h": 40, "default_content": "",
+        "controls": [],
+    },
 }