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/elements/add_tab.py

@@ -0,0 +1,19 @@
+import db
+
+from .add_element import add_element
+from .rename_element import rename_element
+
+
+def add_tab(slug, onglets_id, screen_id):
+    """Ajoute un nouvel onglet à un élément "onglets" : un simple conteneur
+    posé comme enfant (voir render_onglets.py), nommé "Onglet N" par défaut
+    (N = son rang) — ce nom est directement ce qui s'affiche sur l'onglet,
+    modifiable ensuite comme celui de n'importe quel élément."""
+    conn = db.connect(slug)
+    count = conn.execute(
+        "SELECT COUNT(*) AS c FROM _screen_elements WHERE parent_id = ?", (onglets_id,)
+    ).fetchone()["c"]
+    conn.close()
+    tab_id = add_element(slug, screen_id, "conteneur", parent_id=onglets_id)
+    rename_element(slug, tab_id, f"Onglet {count + 1}")
+    return tab_id