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).

screens/__init__.py

@@ -77,6 +77,7 @@ from .elements.add_element import add_element
 from .elements.add_tab import add_tab
 from .elements.update_element_geometry import update_element_geometry
 from .elements.reorder_element import reorder_element
+from .elements.move_element_to_container import move_element_to_container
 from .elements.save_element_controls import save_element_controls
 from .elements.delete_element import delete_element
 from .elements.duplicate_element import duplicate_element

screens/elements/move_element_to_container.py

@@ -0,0 +1,62 @@
+import db
+
+from ..widgets.container_widgets import CONTAINER_WIDGETS
+
+
+def move_element_to_container(slug, element_id, new_parent_id):
+    """Déplace element_id pour qu'il devienne un enfant de new_parent_id
+    (glisser-déposer dans l'arborescence, voir screen_edit.html) — posé en
+    DERNIER enfant, l'ordre précis à l'intérieur se règle ensuite comme
+    d'habitude par glisser-déposer entre frères et sœurs (voir
+    reorder_element.py). Refuse silencieusement (ne touche à rien) si :
+    new_parent_id n'est pas un widget conteneur, si c'est element_id
+    lui-même, ou si c'est un de ses propres descendants (ça créerait un
+    cycle — un conteneur ne peut pas finir posé à l'intérieur de lui-même)."""
+    conn = db.connect(slug)
+    rows = conn.execute(
+        "SELECT id, screen_id, parent_id, widget, z_index FROM _screen_elements WHERE id IN (?, ?)",
+        (element_id, new_parent_id),
+    ).fetchall()
+    by_id = {r["id"]: r for r in rows}
+    el = by_id.get(element_id)
+    target = by_id.get(new_parent_id)
+    if not el or not target or el["screen_id"] != target["screen_id"]:
+        conn.close()
+        return
+    if target["widget"] not in CONTAINER_WIDGETS:
+        conn.close()
+        return
+    if new_parent_id == element_id:
+        conn.close()
+        return
+
+    # Cycle : new_parent_id ne doit pas être element_id, ni un descendant de
+    # element_id (direct ou indirect) — sinon element_id finirait posé à
+    # l'intérieur de sa propre arborescence.
+    children_of = {}
+    for r in conn.execute(
+        "SELECT id, parent_id FROM _screen_elements WHERE screen_id = ?", (el["screen_id"],)
+    ).fetchall():
+        if r["parent_id"]:
+            children_of.setdefault(r["parent_id"], []).append(r["id"])
+    stack = list(children_of.get(element_id, []))
+    descendants = set()
+    while stack:
+        node = stack.pop()
+        if node in descendants:
+            continue
+        descendants.add(node)
+        stack.extend(children_of.get(node, []))
+    if new_parent_id in descendants:
+        conn.close()
+        return
+
+    max_z = conn.execute(
+        "SELECT MAX(z_index) AS m FROM _screen_elements WHERE screen_id = ?", (el["screen_id"],)
+    ).fetchone()["m"]
+    conn.execute(
+        "UPDATE _screen_elements SET parent_id = ?, z_index = ? WHERE id = ?",
+        (new_parent_id, (max_z or 0) + 1, element_id),
+    )
+    conn.commit()
+    conn.close()