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>

static/style.css

@@ -206,6 +206,17 @@ body.builderBody > main.content{ flex:1 1 auto; min-height:0; overflow:hidden; d
 }
 .contextMenu button:hover{ background:var(--panel2); }
 
+/* Élément à onglets (voir render_onglets.py) */
+.forgeTabBar{ display:flex; flex-wrap:wrap; gap:2px; border-bottom:1px solid var(--border); }
+.forgeTabBtn{
+  background:none; border:none; border-bottom:2px solid transparent; color:var(--text-dim);
+  padding:8px 14px; font-size:14px; font-family:inherit; cursor:pointer; white-space:nowrap;
+}
+.forgeTabBtn:hover{ color:var(--text); }
+.forgeTabBtn.active{ color:var(--text); border-bottom-color:var(--accent); font-weight:600; }
+.forgeTabPanels{ min-width:0; }
+.forgeTabPanel{ padding-top:10px; }
+
 .propGroup{ border:1px solid var(--border); border-radius:8px; margin-bottom:8px; background:var(--panel); }
 .propGroup > summary{
   list-style:none; cursor:pointer; padding:9px 12px; font-size:13px; color:var(--accent);