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/rendering/render_element_html.py

@@ -13,6 +13,7 @@ from .render_checkbox_or_radio import _render_checkbox_or_radio
 from .render_repeater import _render_repeater
 from .render_jauge import _render_jauge
 from .render_overlay import _render_overlay
+from .render_onglets import _render_onglets
 
 
 def render_element_html(el, slug=None, children_map=None, ctx=None, parent_flex_direction=None):
@@ -43,6 +44,8 @@ def render_element_html(el, slug=None, children_map=None, ctx=None, parent_flex_
         return _render_jauge(el, meta, slug, children_map, ctx, parent_flex_direction)
     if special == "overlay":
         return _render_overlay(el, meta, slug, children_map, ctx, parent_flex_direction)
+    if special == "onglets":
+        return _render_onglets(el, meta, slug, children_map, ctx, parent_flex_direction)
 
     tag = meta.get("tag")
     if tag is None:  # widget à balise dynamique (ex: "titre" -> h1/h2/h3)