Adds a new "class:" control-target kind (save_element_controls.py, _visible_attrs.py) alongside the existing content/attr/style ones, so a widget can carry CSS classes built from independent named slots (color, size, shape...) without them overwriting each other. Bouton and Titre get their Bulma base class (button/title) via fixed_attrs; Conteneur gets an opt-in "Carte (Bulma)" preset instead of a forced default, since it's also used as an invisible layout wrapper. Bouton's font-size/border-radius sliders no longer freeze their default value into inline style at creation (new c_slider no_freeze flag), so Bulma's own button styling shows through until a user actually customizes it — inline style still wins over any class the moment it's set, exactly like today.
Loads bulma.min.css via CDN in screen_edit.html and play.html, same pattern as animate.css.
william
38f33d88d9