feat(scenes): render the Scene panel's controls from the store

SceneControls no longer takes element handles or writes the Scene panel's
button states, PLAY label, status line, progress bar or runtime chip: it
presents them to the gev-scenes store, and the panel's buttons, scene
select and IMPORT chooser call its actions there. The chooser opens only
while an owner presents the panel and is cleared once its import settles.
On destroy the panel keeps its last status and buttons, without shots or
the runtime chip, as before.

src/scenes/director.js

@@ -395,8 +395,8 @@ export class SceneDirector {
   }
 
   /**
-   * Wire up all scene-panel DOM event listeners and render the initial UI state.
-   * Exits silently if the scene-select element is missing (headless/test mode).
+   * Present the Scene panel through SceneControls, which publishes it to the
+   * store the panel views render; a page without them simply draws nothing.
    */
   _initUI() {
     this._sharing?.mount();