[FIX] web_editor: fix snippets overlay buttons ui

After [1] was merged, the delete/move buttons of the snippets overlay
reduced in size, because of the font-size difference between the backend
and the frontend.

This commit fixes it by setting a fixed font-size of 16px (this restores
the same size as before, as the button was using 1rem on the frontend,
and most browsers use 16px as a default font size).

[1]: https://github.com/odoo/odoo/commit/31cc10b91dc7762e23b4bde9b945be0c4ce3fe3b

task-2687506

Part-of: odoo/odoo#96401

addons/web_editor/static/src/scss/wysiwyg_snippets.scss

@@ -1973,6 +1973,7 @@ body.editor_enable.editor_has_snippets {
         border-color: $o-we-handles-accent-color;
         background: transparent;
         text-align: center;
+        font-size: 16px;
         transition: opacity 400ms linear 0s;
 
         &.o_overlay_hidden {