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/LoadingDots.svelte

@@ -0,0 +1,46 @@
+<script lang="ts">
+  import type { ClassValue } from 'svelte/elements';
+
+  interface Props {
+    class?: ClassValue;
+  }
+
+  let { class: className }: Props = $props();
+</script>
+
+<div class="delayed inline-flex items-center gap-1 {className}">
+  {#each [0, 1, 2] as i (i)}
+    <span class="dot block size-1.5 rounded-full bg-white shadow-[0_0_3px_rgba(0,0,0,0.6)]" style:--delay="{i * 0.25}s"
+    ></span>
+  {/each}
+</div>
+
+<style>
+  .delayed {
+    visibility: hidden;
+    animation: delayed-visibility 0s linear 0.4s forwards;
+  }
+
+  @keyframes delayed-visibility {
+    to {
+      visibility: visible;
+    }
+  }
+
+  .dot {
+    animation: dot-stream 1.6s var(--delay, 0s) ease-in-out infinite;
+  }
+
+  @keyframes dot-stream {
+    0%,
+    80%,
+    100% {
+      opacity: 0.3;
+      transform: scale(0.8);
+    }
+    40% {
+      opacity: 1;
+      transform: scale(1.15);
+    }
+  }
+</style>