Show the wait for the first background effect on its tile

- The first effect chosen in a session shows a spinner on its own tile,
  in place of the tick, until a frame carrying it is drawn. Later changes
  show nothing, having nothing to prepare.
- Ended by the frame, not the promises, which resolve while the segmenter
  is still being built. The transformer reports the first frame that went
  through the segmenter; one passed through untouched doesn't count.
- The pipeline publishes the wait with its state, and both footer view
  models take it as an input.
- The lobby tests' provider mock gains the observable hook, built once: a
  fresh observable per render rebuilt the footer on every render.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/utils/test-viewmodel.ts

@@ -188,6 +188,7 @@ export function getBasicCallViewModelEnvironment(
     vm,
     muteStates,
     mediaDevices,
+    constant(false),
     "reactionId",
     { showControls: true, header: HeaderStyle.Standard },
   );