Move icon gallery to the left panel, drop the redundant Icône tile, add drag-into-container in the tree

- The "icone" widget no longer appears as a generic tile in "Ajouter un élément sur l'écran" / "Ajouter DANS ce conteneur" — the icon gallery (now living under "Ajouter un élément sur l'écran" in the left panel instead of the right one) is the only way to add one, always pre-set to the icon you picked.
- The element tree now supports dragging an element onto a container row to move it inside (last child), from anywhere in the tree — not just reordering within the same parent. Hovering a container row splits it into before/after/inside zones (thirds) when dragging a sibling, or "inside only" when dragging from elsewhere in the tree. New move_element_to_container() rejects non-container targets and cycles (dropping a container into itself or one of its own descendants) silently, mirroring reorder_element's existing safety pattern.

Verified end to end: gallery renders in the left panel with no icone tile in the widget grids, and the move endpoint correctly reparents, rejects a cycle, and rejects a non-container target. Full suite green (89).

routes/elements/__init__.py

@@ -3,6 +3,7 @@ from . import (
     element_add_child,
     element_geometry,
     element_reorder,
+    element_move_to_container,
     element_save,
     element_delete,
     element_duplicate,

routes/elements/element_move_to_container.py

@@ -0,0 +1,21 @@
+from flask import request, jsonify, abort
+
+import screens
+
+from core.flask_app import app
+
+
+@app.route("/game/<slug>/elements/<int:element_id>/move-to-container", methods=["POST"])
+def element_move_to_container(slug, element_id):
+    """Appelé en AJAX quand on glisse un élément DANS l'arborescence pour le
+    déposer sur un autre élément (un conteneur) plutôt qu'au-dessus/en
+    dessous — le fait passer enfant de ce conteneur, en dernière position
+    (voir move_element_to_container, qui refuse aussi silencieusement les
+    cas invalides : cible non-conteneur, ou cycle)."""
+    element = screens.get_element(slug, element_id)
+    if not element:
+        abort(404)
+    data = request.get_json(force=True)
+    new_parent_id = int(data["parent_id"])
+    screens.move_element_to_container(slug, element_id, new_parent_id)
+    return jsonify({"ok": True})