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/utils/container-utils.ts

@@ -5,6 +5,19 @@ export interface ContentMetrics {
   offsetY: number;
 }
 
+export const scaleToCover = (
+  dimensions: { width: number; height: number },
+  container: { width: number; height: number },
+): { width: number; height: number } => {
+  const scaleX = container.width / dimensions.width;
+  const scaleY = container.height / dimensions.height;
+  const scale = Math.max(scaleX, scaleY);
+  return {
+    width: dimensions.width * scale,
+    height: dimensions.height * scale,
+  };
+};
+
 export const scaleToFit = (
   dimensions: { width: number; height: number },
   container: { width: number; height: number },