Redesign the screen editor's element tree and add duplication

The "Éléments de cet écran" tree now renders every level of nesting
(previously stopped after one level of children) as a compact single-line
list, and right-clicking a row opens a context menu to duplicate the
element (and its full subtree) in place, in its current container.

Also: all property panels start collapsed instead of some being open by
default, the redundant nested element list inside "Ajouter DANS ce
conteneur" is removed (it only needs the widget picker), and the
now-unneeded "a container is selected" warning banner is gone.

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

routes/elements/element_duplicate.py

@@ -0,0 +1,14 @@
+from flask import redirect, url_for, abort
+
+import screens
+
+from core.flask_app import app
+
+
+@app.route("/game/<slug>/elements/<int:element_id>/duplicate", methods=["POST"])
+def element_duplicate(slug, element_id):
+    element = screens.get_element(slug, element_id)
+    if not element:
+        abort(404)
+    new_id = screens.duplicate_element(slug, element_id)
+    return redirect(url_for("screen_edit", slug=slug, screen_id=element["screen_id"], selected=new_id))