[FIX] web_editor: fix video size in grid mode

Before this commit, `o_grid_item_image` was not handling videos but image-only.

This commit adds support for `.media_iframe_video` in addition to `img` to
ensure consistent styling across both images and videos within the grid items.

task-3103775

closes odoo/odoo#121266

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/web_editor/static/src/scss/web_editor.frontend.scss

@@ -58,7 +58,7 @@
 }
 
 .o_grid_item_image {
-    > img {
+    > img, > .media_iframe_video {
         width: 100% !important;
         height: 100% !important;
         object-fit: cover !important;