Consolidate the properties panel: Position & taille, no more Taille/Survol/duplicate Disposition

- Merge "Taille dans le conteneur" into "Position & taille": a child
  element (posé dans un conteneur/répéteur/groupe de champs) now shows
  greyed-out X/Y (not applicable, it follows its parent's layout) and
  a single Largeur/Hauteur field per axis with a unit selector (%/px)
  instead of the old fixed-px-only sliders capped at 1000 — which was
  the root cause of a landscape-mode bug where a child couldn't be
  made wider than 1000px even though the actual screen was wider.
  New c_size control type (one style key per axis, not two competing
  sliders — an earlier px+% two-slider attempt let the untouched
  slider silently clobber the other's value on every autosave).
- Merge the two "Disposition" groups (visibility + scale, previously
  split apart in UNIVERSAL_CONTROLS by unrelated groups) into one.
- Remove the "Survol" panel: hovering is conceptually a flow trigger,
  not a static element property — to be reintroduced there. The
  underlying data-hover-text/bindHoverTexts runtime is untouched.

screens/elements/save_element_controls.py

@@ -64,6 +64,25 @@ def save_element_controls(slug, element_id, form):
                 style["transform"] = f"scale({val})"
             continue
 
+        if ctype == "size":
+            # Un seul champ CSS ("width"/"height") mais deux entrées de
+            # formulaire (valeur + unité) — voir size_override_controls.py
+            # pour pourquoi ce n'est PAS deux contrôles slider séparés.
+            _kind, name = control["target"].split(":", 1)
+            try:
+                num = float(form.get(field_name + "_value") or 0)
+            except (TypeError, ValueError):
+                num = 0.0
+            unit = form.get(field_name + "_unit", "%")
+            if unit not in ("px", "%"):
+                unit = "%"
+            if num <= 0:
+                style.pop(name, None)
+            else:
+                num_str = str(int(num)) if num == int(num) else str(num)
+                style[name] = f"{num_str}{unit}"
+            continue
+
         target = control["target"]
         if ctype == "checkbox":
             checked = form.get(field_name) is not None