Make Bulma the default style engine for Bouton/Titre/Conteneur, layered under existing inline customization

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.

screens/rendering/visible_attrs.py

@@ -3,11 +3,23 @@ from .auto_id import _auto_id
 
 
 def _visible_attrs(el, meta, ctx=None):
+    raw_attrs = el.get("attributes") or {}
     attrs = {
         k: (_apply_ctx(v, ctx) if isinstance(v, str) else v)
-        for k, v in (el.get("attributes") or {}).items() if not k.startswith("_")
+        for k, v in raw_attrs.items() if not k.startswith("_")
     }
     attrs.update(meta.get("fixed_attrs", {}))
+    # Classes externes (ex. Bulma) réglées via un contrôle "class:xxx" — voir
+    # save_element_controls.py — chacune mémorisée sous "_class_xxx" pour ne
+    # pas s'écraser entre elles ; assemblées ici avec la classe de base du
+    # widget (posée via fixed_attrs, ex. "button"/"box"/"title") pour former
+    # l'attribut class final. Une classe personnalisée éventuellement tapée
+    # à la main (peu probable ici, mais possible via fixed_attrs) est gardée
+    # en tête.
+    extra_classes = [v for k, v in raw_attrs.items() if k.startswith("_class_") and v]
+    if extra_classes:
+        base_class = attrs.get("class", "")
+        attrs["class"] = " ".join(([base_class] if base_class else []) + extra_classes)
     attrs.setdefault("id", _auto_id(el))
     attrs["data-element-id"] = el["id"]
     if el.get("parent_id"):