fix(web): stale adaptive image when original overlays preview (#27621)

AdaptiveImage stacks quality layers (thumbnail → preview → original) as they load. Without compositor layer promotion on the container, the browser could render a stale frame when the original-quality layer was overlaid on top of the preview-quality layer.

Add `will-change: transform` as a class on AdaptiveImage's root element so it gets a dedicated compositor layer from first paint. This also subsumes the imperative `style.willChange = 'transform'` that zoomImageAction was applying to the same element (the zoom target from photo-viewer is the AdaptiveImage root), so drop that now-redundant code.

Change-Id: Icd866a2bb5a5fce299c36404547fa0546a6a6964

web/src/lib/actions/zoom-image.ts

@@ -124,9 +124,6 @@ export const zoomImageAction = (node: HTMLElement, options?: { zoomTarget?: HTML
     { capture: true, signal },
   );
 
-  if (options?.zoomTarget) {
-    options.zoomTarget.style.willChange = 'transform';
-  }
   node.style.overflow = 'visible';
   node.style.touchAction = 'none';
   return {
@@ -138,9 +135,6 @@ export const zoomImageAction = (node: HTMLElement, options?: { zoomTarget?: HTML
     },
     destroy() {
       controller.abort();
-      if (options?.zoomTarget) {
-        options.zoomTarget.style.willChange = '';
-      }
       for (const unsubscribe of unsubscribes) {
         unsubscribe();
       }

web/src/lib/components/AdaptiveImage.svelte

@@ -148,7 +148,7 @@
   });
 </script>
 
-<div class="relative h-full w-full overflow-hidden will-change-transform" bind:this={ref}>
+<div class="relative h-full w-full overflow-hidden" bind:this={ref}>
   {@render backdrop?.()}
 
   <div