HEEx render template for the OG editor. Split out from the LV module so the event-handler code stays scannable.

The layout is:

┌───────────────────────────────────────────────────┐
│  Toolbar: name | save state | insert | save        │
├──────────────────────────────┬────────────────────┤
│  Platform tabs + raw toggle  │  Slots used        │
│  OpenFresco.Editor stage     │  Selected props /  │
│  inside the platform frame   │  template props    │
└──────────────────────────────┴────────────────────┘

The stage is OpenFresco's LiveComponent; everything around it is ours. Property-panel forms all post update_prop/update_canvas events handled by the LV through PhoenixKitOG.SceneEdit.