feat: adaptive progressive image loading for photo viewer (#26636)

* feat(web): adaptive progressive image loading for photo viewer

Replace ImageManager with a new AdaptiveImageLoader that progressively
loads images through quality tiers (thumbnail → preview → original).

New components and utilities:
- AdaptiveImage: layered image renderer with thumbhash, thumbnail,
  preview, and original layers with visibility managed by load state
- AdaptiveImageLoader: state machine driving the quality progression
  with per-quality callbacks and error handling
- ImageLayer/Image: low-level image elements with load/error lifecycle
- PreloadManager: preloads adjacent assets for instant navigation
- AlphaBackground/DelayedLoadingSpinner: loading state UI

Zoom is handled via a derived CSS transform applied to the content
wrapper in AdaptiveImage, with the zoom library (zoomTarget: null)
only tracking state without manipulating the DOM directly.

Also adds scaleToCover to container-utils and getAssetUrls to utils.

* fix: don't partially render images in firefox

* add passive loading indicator to asset-viewer

---------

Co-authored-by: Alex <alex.tran1502@gmail.com>

web/src/lib/components/ImageLayer.svelte

@@ -0,0 +1,47 @@
+<script lang="ts">
+  import Image from '$lib/components/Image.svelte';
+  import type { AdaptiveImageLoader, ImageQuality } from '$lib/utils/adaptive-image-loader.svelte';
+  import type { Snippet } from 'svelte';
+
+  type Props = {
+    adaptiveImageLoader: AdaptiveImageLoader;
+    quality: ImageQuality;
+    src: string | undefined;
+    alt?: string;
+    role?: string;
+    ref?: HTMLImageElement;
+    width: string;
+    height: string;
+    overlays?: Snippet;
+  };
+
+  let {
+    adaptiveImageLoader,
+    quality,
+    src,
+    alt = '',
+    role,
+    ref = $bindable(),
+    width,
+    height,
+    overlays,
+  }: Props = $props();
+</script>
+
+{#key adaptiveImageLoader}
+  <div class="absolute top-0" style:width style:height>
+    <Image
+      {src}
+      onStart={() => adaptiveImageLoader.onStart(quality)}
+      onLoad={() => adaptiveImageLoader.onLoad(quality)}
+      onError={() => adaptiveImageLoader.onError(quality)}
+      bind:ref
+      class="h-full w-full bg-transparent"
+      {alt}
+      {role}
+      draggable={false}
+      data-testid={quality}
+    />
+    {@render overlays?.()}
+  </div>
+{/key}