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/widgets/control_value.py

@@ -23,6 +23,14 @@ def _control_value(el, control):
         m = re.search(r"scale\(([\d.]+)\)", transform)
         return float(m.group(1)) if m else control.get("default", 1)
 
+    if ctype == "size":
+        _kind, name = control["target"].split(":", 1)
+        raw = style.get(name)
+        m = re.match(r"^([\d.]+)(px|%)$", str(raw or ""))
+        if m:
+            return {"value": float(m.group(1)), "unit": m.group(2)}
+        return {"value": control.get("default", 0), "unit": "%"}
+
     target = control["target"]
     if target == "content":
         return el.get("content") or control.get("default", "")