Add a Container tab and propagate template edits

The Container tab reuses SceneViewport against any object holder, so a
container template is edited exactly like a scene. Right-click no longer
changes the selection, which previously made nesting impossible: opening
the menu on a container loaded it, and "Place in container" then targeted
itself.

Template edits (move, rename, override) now reach already-placed instances
per slot, through a shared templateSync used by both the apply path and the
plain object commands. Copies the user has touched keep their own value.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tools/e2e/05-drawing.mjs

@@ -30,7 +30,7 @@ export async function drawing(t) {
     JSON.stringify(await orderOfCrates()) === '["o1","o2"]',
     JSON.stringify(await orderOfCrates()),
   );
-  await select({ kind: 'object', sceneId: 'scene1', objectId: 'o1' });
+  await select({ kind: 'object', holderId: 'scene1', objectId: 'o1' });
   await sleep(200);
   const zLabel = await locate('Z');
   await click(zLabel.x + 160, zLabel.y); // plain click → text editing with everything selected