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/layout-utils.spec.ts

@@ -0,0 +1,54 @@
+import { scaleToFit } from '$lib/utils/container-utils';
+
+describe('scaleToFit', () => {
+  const tests = [
+    {
+      name: 'landscape image in square container',
+      dimensions: { width: 2000, height: 1000 },
+      container: { width: 500, height: 500 },
+      expected: { width: 500, height: 250 },
+    },
+    {
+      name: 'portrait image in square container',
+      dimensions: { width: 1000, height: 2000 },
+      container: { width: 500, height: 500 },
+      expected: { width: 250, height: 500 },
+    },
+    {
+      name: 'square image in square container',
+      dimensions: { width: 1000, height: 1000 },
+      container: { width: 500, height: 500 },
+      expected: { width: 500, height: 500 },
+    },
+    {
+      name: 'landscape image in landscape container',
+      dimensions: { width: 1600, height: 900 },
+      container: { width: 800, height: 600 },
+      expected: { width: 800, height: 450 },
+    },
+    {
+      name: 'portrait image in portrait container',
+      dimensions: { width: 900, height: 1600 },
+      container: { width: 600, height: 800 },
+      expected: { width: 450, height: 800 },
+    },
+    {
+      name: 'image matches container exactly',
+      dimensions: { width: 500, height: 300 },
+      container: { width: 500, height: 300 },
+      expected: { width: 500, height: 300 },
+    },
+    {
+      name: 'image smaller than container scales up',
+      dimensions: { width: 100, height: 50 },
+      container: { width: 400, height: 400 },
+      expected: { width: 400, height: 200 },
+    },
+  ];
+
+  for (const { name, dimensions, container, expected } of tests) {
+    it(`should handle ${name}`, () => {
+      expect(scaleToFit(dimensions, container)).toEqual(expected);
+    });
+  }
+});