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/components/CallFooter.tsx

@@ -87,6 +87,8 @@ export interface FooterState {
   backgroundEffects: BackgroundEffectChoice[];
   /** What the user is told about effects here, if anything. */
   backgroundEffectNotice: "unavailable" | "slow" | undefined;
+  /** Whether the first effect chosen is still being prepared. */
+  backgroundEffectSettling: boolean;
   showFooter: boolean;
 
   /* This is needed for WindowMode = "flat" */
@@ -173,6 +175,7 @@ export const CallFooter: FC<FooterProps> = ({
   const backgroundEffect = useBehavior(vm.backgroundEffect$);
   const selectBackgroundEffect = useBehavior(vm.selectBackgroundEffect$);
   const backgroundEffectNotice = useBehavior(vm.backgroundEffectNotice$);
+  const backgroundEffectSettling = useBehavior(vm.backgroundEffectSettling$);
   const backgroundEffects = useBackgroundEffectLabels(
     useBehavior(vm.backgroundEffects$),
   );
@@ -240,6 +243,7 @@ export const CallFooter: FC<FooterProps> = ({
         backgroundEffects={backgroundEffects}
         selectedBackgroundEffect={backgroundEffect}
         onSelectBackgroundEffect={selectBackgroundEffect}
+        backgroundEffectSettling={backgroundEffectSettling}
         backgroundEffectNotice={
           backgroundEffectNotice === "unavailable"
             ? t("background_effects.unavailable")